All spoilers on this page are left unmarked. You Have Been Warned!
In Avengers: Infinity War, the main goal of Thanos is that once he obtains all six Infinity Stones, he will use it to wipe out half of the Universe. Thanos's belief is that by culling half of the population, the species can survive as he proves with Gamora's homeworld. However the idea of removing half goes against Chinese Taoism.
In Chinese Taoism, there exists a balance of Yin and Yang, Light and Dark. One half is needed for the other to survive. When one is removed, there is imbalance. The theme of Chinese Taoism is prevalent throughout the movie. Each character has someone to complement the other.
Steve Rogers/Captain America has Bucky. Two friends now together again.
Tony Stark/Iron Man has Spider-Man. A mentor and his student.
Thor and Loki. Two Brothers reunited.
Rocket and Groot. Partners and Best Friends.
Peter Quill/Starlord and Gamora. Lovers.
Black Panther and Okoye. A King and the aide.
Vision with Wanda Maximoff. Lovers.
And the list goes on.
However due to Thanos's actions, he not only destroyed his enemies but he ruins himself. During his search for the Soul Stone, in his quest for his mad dream, he had to sacrifice the one he truly loves, his adopted daughter; Gamora. Gamora was Thanos's half. The theme of Yin and Yang reflects this in his last vision with Gamora when she was a child, the vision of Gamora asks what did his victory cost and Thanos replies, "everything." Thanos won but is now imbalanced. Steve lost the friend he worked so hard to get back, Tony lost his protege who was like a son to him and so on. By the end of the film, the balance has been disrupted and it needs to be restored.
If Infinity War is the Yin, the dark, it needs to have the Yang, the light to restore everything. Where ever Captain Marvel is, she'll be there. Infinity War is just Part One of the grander story and Part Two will be there. Infinity War has just begun.
Sacrifice appears to be the central theme of the movie. Six characters have to make a choice between an Infinity Stone (power to do the "greater good") and another person.
Two characters choose to sacrifice the Stone or its location for their sibling (Loki for Thor, Gamora for Nebula) as an Act of True Love, and as a result both suffer death at the hand of Thanos. One character gives up the Stone for the life of an ally (Doctor Strange for Tony Stark) as a part of a Batman Gambit and in the end turns to ash. These sacrifices are accepted: Thanos spares all three victims (Thor, Nebula, Tony), and all of them also survive the Snap.
Two characters choose to sacrifice a loved one for the Stone at the other's request (Peter Quill attempts to kill Gamora, Wanda — Vision). Both sacrifices are praised by Thanos but rejected: the reality turns out distorted by the Reality Stone, or the time is reversed by the Time Stone. Thanos gets both Stones and kills both victims (Gamora, Vision) anyway. In the end, those who made the choice (Quill, Wanda) also turn to ash.
Finally, Thanos himself chooses to sacrifice his daughter (Gamora) for the Stone without her consent. However, as Gamora puts it: "This isn't love". While Thanos appears to succeed horribly, this decision may very well lead to his demise.
The directors of the film stated that some of its major questions are: "What it costs to be a hero in a complicated world" and "Does the value of doing what's right outweigh the cost?" Thanos is an utilitarian who believes that he does what he has to do to balance the supply (resources) and demand (population) and thus to achieve the greater good on a universal scale. From his standpoint, inability to sacrifice personal attachments to meet The Needs of the Many is a weakness and a sign of the lack of will.
Steve Rogers insists on the opposite: "We don't trade lives" of others (they are not interchangeable), we can only lay down our own. Per Dostoevsky : "The higher harmony is not worth the tears of one tortured child." From this standpoint, Thanos is a murderer "on a scale hitherto undreamt of", the characters who refused to trade the lives of their siblings for an abstract "greater good" and died for them are morally justified in their choice and have performed a Heroic Sacrifice, and all the rest (Doctor Strange, Quill, Wanda) are in the grey area. (Quill having sworn on his mother's memory to fulfill his promise to Gamora; and Wanda helping Vision to make his Heroic Sacrifice after he assures her that she could never hurt him).
Take your pick. Sliding Scale of Idealism Versus Cynicism also applies. Note how those who made the most idealistic choice (Loki and Gamora) were punished for it the most, while the most cynical choice (Thanos's) bore most fruit. Or did it?
Thanos is a Well-Intentioned Extremist who firmly believes that he is doing something honorable and selfless. He’s experienced the horrors that are possible when demand for resources overwhelms the available supply, but it never occurs to him that there may be more reasonable ways to resolve an Overpopulation Crisis. Even with the Gauntlet, which essentially makes him omnipotent, it doesn’t occur to him to increase the supply of resources or change the fertility rate. He refuses to listen to anyone else’s ideas and won’t adapt his thought process to accommodate new knowledge. It’s no coincidence that the beautiful pastoral landscape Thanos is transported to at the end closely resembles the vision that he showed the heroes of Titan before it was destroyed. Thanos is trapped by the pain of his past, rendering him unable to move on and grow. In the film’s closing minutes, he forces that same block onto the entire universe.
The most literal example of this idea happens when Thanos murders his favorite daughter, but there are several scattered throughout the film. Tony Stark and Pepper Potts discuss having a child together. Peter Quill’s anger and immaturity gets the better of him when he winds up helping Thanos escape Mantis’ grip. Wakanda as a nation went against centuries of tradition by revealing themselves to the world, and find themselves dealing with an alien invasion soon after. Tony Stark makes Spider-Man an Avenger. Teen Genius Shuri identifies a key aspect of Vision’s design that never even crossed the mind of Bruce Banner, the man who helped create him. It’s even Played for Laughs when Peter Parker and Peter Quill debate whether or not Foot Loose is a classic movie, or when the Guardians scold Groot, who’s acting like a petty teenager.
To quote Screen Prism’s video on the film, the key reason why the cliffhanger ending affects the audience on such a visceral level is the fact that “the wrong characters disappear”. The majority of fans went into this movie expecting to see characters who’ve been part of the MCU for a while now be killed. Instead, all of the original Avengers are left standing, while much of the new guard, (in other words, the future of the Marvel Cinematic Universe), turns to dust. Of course, it’s highly unlikely the deaths we saw will last past Avengers: Endgame. Not only do many of the dead have confirmed roles in upcoming movies, they’re necessary for the broader theme. The MCU is growing up, and for this franchise to have a future, so must its characters. But as for right now, this crisis belongs to the old guard. It’s up to them to assemble and set things right, to assure that they can pass the torch to the next generation of heroes.
Thanos's motivations are clear from the start of Infinity War; wanting to protect the universe from another extinction event like on Titan. It's also clear that Thanos feels like this is a necessary evil, but it's not immediately clear why he feels like he has to be the one to carry it out. To understand this, we only have to look at how he interacts with the people around him.
Thanos still refers to Gamora as "little one", even though she's already well in her twenties. He calls Wanda — another woman in her twenties — "my child". His followers (and victims, judging by Ebony Maw's speech to the Asgardians) are called "the Children of Thanos."
Thanos sees the universe as children, and himself as the parent.
Thanos feels like he has to be the responsible one, because the rest of the universe refuses to be. He doesn't see the resistance posed by the Guardians and the Avengers as a clash between ideologies, but rather as a child throwing a tantrum as he's dragging them to the dentist. He has to do the right thing, whether the people affected like it or not.
Now contrast this to Tony Stark.
Tony's relationship to Peter Parker can only be described as paternal. It's a relationship that started in Captain America: Civil War, was built up further in Spider-Man: Homecoming, and is felt all throughout Infinity War. Not only that, Tony's interactions with Mantis, Star-lord, Drax and (arguably) Doctor Strange feel like an exasperated father trying to deal with a bunch of hyperactive children.
However, the big difference between Tony and Thanos is how they treat their respective "kids."
Thanos sees everyone around him as beneath him, too stupid and childish to know any better, leaving him as the responsible adult.
Tony, on the other hand, sees Peter and the Guardians as his equals. While he still treats them — Peter especially — like children ("The adults are talking"), he doesn't act like they're incapable of holding their own, be it physically or mentally.
Tony lets Peter come up with a plan to save Doctor Strange, which he then helps carry out, choosing it over any plans he might have had. He lets Peter help steer the Q-ship, and instantly listens when Peter tells him to turn. He doesn't get angry at Drax for not listening to his plans ("Okay, Mr. Clean's on his own page"). His first impulse when Strange panics when coming out of his trance is to try and calm him down. He doesn't try to pull Peter out of the fight like in Civil War, because he knows Peter can handle himself. He tries to calm Star-lord down after they learn about Gamora's death, and again when Mantis and Drax fall to dust. And when Strange chooses to sacrifice the Time Stone in exchange for Tony's life, he doesn't yell, instead opting for a quiet "Why would you do that?"
And when Tony and Thanos finally go toe-to-toe, their pre- and post-fight talk goes a bit different than what one might have come to expect.
Tony: You know me?
Thanos: I do. You're not the only one cursed with knowledge.
Thanos: You have my respect, Stark. When I'm done, half of humanity will still be alive. I hope they remember you.
Thanos doesn't treat Tony like a child, but as another adult, his equal. And in a way, Tony is.
Just like it is no coincidence that everything surrounding Thanos seems to be child-themed, it's also no coincidence that the scene introducing Tony into Infinity War is the one where he opens up to Pepper about wanting to have a baby with her. Tony and Thanos are two sides of the same coin. Thanos embodies the toxic and abusive side of parenthood in his treatment of Gamora and Nebula, while Tony embodies the healthy and supportive side in his relationship with Peter and the Guardians.
And at the end of Infinity War, both of them have lost their surrogate children. However, while Thanos himself killed Gamora for what he perceived as the greater good, Tony had to watch Peter slowly die in his arms while he couldn't do anything to save him.
One can only wonder how that will affect them, and the second, inevitable clash between them.
In an attempt to pin down abstract forces outside their control, Ancient Greeks came up with their many Anthropomorphic Personifications, such as Chronos for time, Ananke for destiny, Eros for love and Thanatos for death. Centuries later, Sigmund Freud suggested that all humans share a life instinct and a death drive, commonly called after the Greek gods — Eros and Thanatos. When in 1970s Jim Starlin attended psychology classes, the idea served as an inspiration for his new comic book character — the Mad Titan Thanos. In the comics, Thanos was in love with Lady Death and sowed destruction in an attempt to please her.
In Infinity War, Thanos is free of such concerns. He sees himself as an impersonal force that acts out of necessity, and thus is Ananke in the flesh. Not a petty god who craves worship but more than a god. (Hence his smirk at Loki's parting shot at him) And in his mind, death is the necessity, justifying his name. He convinced himself that he must kill half the universe, and the prospect neither pleases nor disturbs him. As he tells Doctor Strange, sparing the other half is him showing mercy.
The heroes who oppose him, however, are very much human (even when they are talking raccoons). They want to live, and they share invisible bonds with one another that make it worth living — those of love. And true to the source, "eros" and "thanatos" are intertwined.
Love kills — Thanos uses Loki's and Gamora's feelings for their siblings to coerce them into giving up an Infinity Stone or its location, and then murders both of them. Love makes one kill — Gamora appeals to Peter Quill's feelings for her and his mother to ensure that he kills her, and Thor seeks revenge for a personal loss. Death triumphs over love — despite all her struggles Wanda has to kill Vision, Thanos forswears his last glimpse of humanity by tossing Gamora off the cliff, and in the end those who are dear to heroes vanish into thin air.
However, the final part of this interplay is...less clear — the one where love saves. There are numerous smaller moments where the power of Philia (friendship) helps to preserve people. Heimdall rescues Bruce Banner from Thanos, and though he is killed for his act, Bruce lives. Loki gives up the Space Stone, but in doing so, saves Thor's life. Gamora risks the universe, but finally "takes a fall" for Nebula, and lets her live. Groot gives up his arm, but in doing so creates the weapon that saves Thor's life. Vision has plenty of time to escape Corvus Glave, but being on the side of life chooses to save Captain America. Small sacrifices, undertaken by love, and those who were sacrificed for live beyond Thanos's cruel march.
But in the grand scale of things, heroes fail to prevent Thanos from gaining the Stones, to take the Gauntlet from him, to extract revenge and ultimately to protect the universe. The true horror of the film comes from the realization that love seems powerless in the face of death, and it cannot overcome.
At least, not in the climax of a two chapter story. Only time will tell if love can take its one chance out of 14 million and conquer all in the end.
Rhodes: That's right. And I'm pretty sure I've paid for it.
Ross: You having second Thoughts?
Rhodes: Not anymore.
James Rhodes believed in the government. He believed in discipline, hiearchy, strategy and the well intentions of government structures. This strict view was helpful in restraining his less inclined partner Iron Man. And in the end, that devotion gave him a boost as both War Machine and Iron Patriot. His dedicated service helped to protect his country time and again from Vanko, The Mandarin, and Ultron. It also lead to his outright support of the Sokovia Accords. Thinking having supervision was exactly what they needed.
And then he got crippled in battle. The act itself wasn't what shocked him. He even told Tony he knew the risks every time he went into combat zones. But then after 2 years of stalling, Rhodes was forced into a decision he didn't believe he'd have to make. The UN still refused to bring back the rogue avengers when Thanos was looming large, forcing Rhodey to make his decision To Be Lawful or Good. But at his core, Rhodes is still a serviceman above all else. And with little hesitation, he welcomed his friends back. He may no longer have the structures of government to back him up from here on out, but the Military Superhero finally had that final shift Captain America had back in the day in thinking of the greater good through the power of his team rather than sticking to organizations.
The Hulk is the first hero we see fight Thanos one on one. The "Strongest Avenger" pummeled Thanos, but in the end proved no match for Thanos's own strength and experience, only being rescued at the last moment by Heimdall.
Bruce's first role in the movie then is that of Prophet. Giving his first warning to Dr. Strange: "Thanos is coming. He's Coming!" His position then gives him credibility to then warn his old friend Tony Stark of the oncoming storm. And in addition, serve as the Only Sane Man to Tony's hesitance to recontact Steve Rogers. Emphasizing that with Thor gone and such a great threat coming, all their ego has to be put aside for the sake of the universe.
But before the move can be made, then The Black Order arrives, and Bruce tries to bring out his big guns again. Only...Hulk won't come out. Though the fans and directors saw the reason for this differently, for the sake of simplicity, we'll just say that Hulk won't be Bruce's easy weapon for his own reasons. And this stuns Bruce, as after Thor: Ragnarok, you'd think that Hulk would be happy to get out in control again after being in the drivers seat for 2 years. But no, Hulk simply refused to come forth.
Linking up and making the call Tony failed to make, Bruce's next role was as consultant. Pushing that it'd be possible to get the Mind Stone out of Vision and he'd still be able to live on. That it wasn't necessary for him to sacrifice himself...yet.
Flying into Wakanda, the forces of Thanos came in hard yet again, and this time, Bruce was pushed into a role he never would've expected: As a true hero. Wanting to help out in the defense of Wakanda, Bruce donned The Hulkbuster, and fought alongside everyone else. Making some tactical assesments during the battle, and even being the first on the scene to protect Vision when Corvus Glaive made his move.
And it's here that Bruce truly found a new side to himself. He was terrified without his friends to fight alongside him, and again, called out for the Hulk to save them like he always did "at the last second". But the Hulk would only say "NOOOO!!!!" So Bruce, in frustration, mounted his courage again, and using a little ingenuity and determination, defeated Thanos's power fighter. He had finally proven himself as much the hero as the Hulk was. And in doing so, merely muttered to himself, "Hulk, we got a lot to figure out pal." Bruce had become as mentally strong as his other self, and perhaps in doing so, sets himself up for one final journey in the ruins that Thanos created: finding true harmony between him and Hulk.
T'Challa promised to open Wakanda up to help the world "as one tribe." That after his incident with his cousin N'Jaka, T'Challa saw that Wakanda needed to be the compassionate, caring nation it never had been for so long.
And lo and behold, he receives his opportunity by repaying a debt he owed Captain America. The nation did much to give Bucky a partial retirement to peace, but he decided to go even further by offering his sister's expertise to accomplish their goals of getting Vision's stone destroyed and removed safely. And with the coming of the Order, he was ready to lead from the front in asserting his sovereignty with words and fists.
And indeed, the Wakandan army suffered the most for trying to protect the life of one, and in doing so, many. The standing army, border tribe, Dora Milage, and Jabari all united for this action of life, and the brutal battle that resulted weakened, but never broke Wakanda's military. It could even be argued that through their efforts, they technically won the conventional battle. And yet, for all their strength, though they could stop the armies, Thanos still managed to get his stones, and leave nothing but dust in his wake. It didn't matter that he'd opened up to helping others, or struggling mightily. In the end, T'Challa crumpled to dust just the same as half of his remaining warriors.
Vision and Wanda first bonded with each other as the true outsiders of the Avengers. Wanda tried to destroy their team, but in doing so lost her brother and home. And Vision, a unique android given life by an AI, magic lightning and the Mind Stone, is a truly unique being. So, in time, they bonded with each other, seeing the hero underneath the rough edges, and eventually falling in love. Even during a rough patch in their relationship, 2 years on, and they're now living together in secrecy. Vision even starts pushing Wanda, saying that while she may have a duty to Cap's secret avengers, what they do together works, and he wants them to stay together.
And then destiny arrives, and everything is thrown into chaos. Vision is attacked by Thanos's generals, critically "wounding" him, rendering his abilities far weaker than before. Wanda fights her hardest to protect him, refusing to give him up, and after being rescued, it seems things might be okay. Only, on hearing the threat Thanos poses, Vision's mechanical way of thinking comest to a quick logical conclusion: If Thanos needs the stone to destroy half the universe, then the quickest way to solve this would be to destroy the stone in his head.
But unfortunately, the only being who he knows could do this is Wanda, and she is adamant in Love that she will not kill the last significant emotional figure in her life. The others push to give them more time in Wakanda, and again, it seems that things might work out. Though Thanos's children come for them again, the love his team has protects him, Wanda staying by his side so that the moment they're ready, she can blow the stone and protect Vision from death. But then Proxima and the others deploy attack vehicles, and realizing that she has the power and thus the responsibility to protect the people out there, Wanda leaves to disable the rolling wheels. Only to then have Corvus sneak into the lab to take Vision all over again.
In their individual fights with the Black Order, Wanda and Vision find themselves in need of rescue. The most powerful avengers forced to be protected by Black Widow, Okoye, Bruce Banner and Captain America. And in both these situations, after the initial rescue, both prove their love and duty to save lives by defeating a member of the order on their own. Wanda by flinging Proxima into a war machine, and Vision by protecting Captain America, even giving him an affirmation that, "We don't trade lives, Captain."
But then Thanos arrives in person, and as they watch, The Mad Titan begins to walk through all the other heroes, and Vision begs Wanda to do what has to be done for the sake of the universe. Recognizing that she shouldn't have to be the one to end him, but that despite their love, their duty had to come first. And despite her immense emotional pain, The Scarlet Witch complied, pouring all her power into holding off Thanos and shattering the Mind Stone. Giving Vision a proper Heroic Sacrifice.
Only, then to Wanda's horror, Thanos uses his new Time stone to reverse her actions, batting her aside and prying the Mind Stone from his head. Taking that which pulled him so close to being human, leaving nothing behind but a grey husk. Only the machine remained, and the stone provided the calculator for Thanos's cruel snap. Wanda soon followed Vision, as she was selected by the snap to become dust. So it was that 2 of the most powerful avengers, despite their best efforts to adhere to their love and their duty, both died despite their best hopes, failing to fulfill either.
Drax: Including my own.
Chiefly speaking, these two were always destined to be in this movie regardless of what everyone else intended. Because both Gamora and Drax had their lives defined by Thanos. And so this movie of confrontation is where both of them are forced to face their destinies head on, only to fail again and again in trying to thwart him.
Drax's culmination here is simple: Thanos, ordering the massacre of his planet, caused the death of his wife and daughter. After learning the truth about how Ronan was only a puppet, Drax swore that one day he'd kill the mad titan for his cruelty. He found new avenues to begin to move on thanks to the guardians giving him a new family, but still Thanos loomed in the back of his mind. And when he sees him in person for the first time, his rage begins to take over. And even when Quill begs for him to think, to see that if they get the reality stone first, then they can take him on more safely, but the old wound was made anew, and Drax had to be mentally sedated before ruining their strategy. Of course, then Thanos revealed he already got the stone, which finally gave Drax the excuse to just attack without restraint, only to be foiled again and again by Thanos's cunning and power at his disposal. And in the end, he was dusted, unable to sate his vengeance.
Gamora, as Thanos's "daughter" has a far more personal, more tragic component to her arc in the movie. Gamora, being raised by Thanos, was partially molded into a woman with an iron will, and in that way, became determined to kill Thanos or die trying the moment Thor reveals that Thanos is now going directly for the infinity stones. But at the same time, Thanos did partially raise her, so there's some lingering affection she has for him, hence when she thinks that she's finally killed him, she breaks down sobbing in relief and grief. Only to then be captured after her boyfriend failed to kill her. (more on that below) She's continually ground against by Thanos, cajoled, argued with, and ultimately, forced to bend to buy time for herself and her sister.
And it's in the last stage of her part of the movie that Gamora's character culmination comes to its tragic, heartbreaking end. For on Vormir, The Red Skull tells Thanos that to gain the stone, he would have to sacrifice what he loves most. A soul for a soul. Gamora at this laughed, thinking that she'd finally gotten her vengeance on Thanos, seeing him halted for loving nothing, only then to see him turn around, determined to kill her, because she IS what he loves most in his own mind. With his iron will to act, Gamora tried one last play: Heroic Suicide by Thanos's own knife. Only, with the power of reality at his hand, Thanos dissolves the knife and then hurls her off the cliff. A sudden but tragic end to the fiercest woman in the galaxy, brought down by her father despite her best efforts.
Nebula and Gamora had a troubled upbringing together. Nebula grew to hate and resent her "older" sister due to her being turned into a near machine with her constant failures in combat. And despite the scraps, ultimately what Nebula wanted from Gamora was an indication that she wasn't alone in Thanos's care. As she said, "I just wanted a sister!" It took them nearly dying in the battle with Ego before they could even get to speaking terms, and the two of them finally understanding each other's anger. Gamora even gives Nebula a chance to start over with Gamora's new family, but alas, Nebula's hatred for Thanos overruled any impulse to begin anew. And with a heartfelt hug goodbye, they parted way.
However, in Infinity War, we find that Nebula did follow through on her desires, and almost assassinated Thanos before being caught. And after being caught, she was forced into the role both daughters hate above all: nothing but a tool for Thanos to get what he wants. A hostage to persuade Gamora to give up the Soul Stone after Thanos found out that she knew it's location. Even in the midst of this torture, Nebula still pleaded with Gamora silently to never give in, and just allow her to die for the Universe. But, after finally making up, this was the sacrifice Gamora wasn't ready to make yet, and thus saved Nebula's life, though the cost would be steep: Vorimir, and a walk to the stone.
For her part, Nebula, despite not being a main character, still wasn't ready to give up once she knew her sister's danger. When Thanos agrees to put her back together for Gamora's sake, Nebula breaks out, and gives the Guardians a heads up that Thanos will reach Titan next, and that they need to intercept him before any of the Black Order return with another stone. But on her arrival, and seeing Thanos in anguish, Nebula puts it all together: "He took her to Vormir. He came back with the soul stone. She didn't." And this shakes her to the core so deeply, she becomes paralyzed. Unable to react in time for Thanos to escape capture. And yet, somehow, she manages to keep going, and is one of the cursed heroes to survive the snap her father wanted. Knowing just as much as Gamora in Thanos's desires, all she can quietly say is, "He did it." Where her journey from here is unclear at the moment, but having been pushed to her lowest like Stark, one can only hope she recovers that will to face Thanos one last time.
Thanos: ...I had to...
Peter: No you didn't.. No you didn't... [then with a yell and a whip of his pistol] NO YOU DIDN'T!
Star Lord had deep seeded issues with facing loss. It started from when he lost his mother, and he just kept running, trying to bury that loss under a masculine persona, wit and snark. Then he found a new family. The Guardians of the Galaxy forced Peter to take on not only leadership, but also responsibility, empathy, and finally get some closure from his mother's loss through interacting with Gamora.
When he got a chance to rebuild his blood family with Ego, he was hesitant at first, but a temptation for power pushed him closer, hoping to combine a spark of normalcy with extraordinary blessings. Then Ego revealed he was responsible for his mom's tumor, and Peter unleashed his anger at once, waking up to the evil his Archnemesis Dad was about to unleash. He was ready to sacrifice himself for his Guardians, only to instead finally see that the man who raised him, whom he had issues with, still cared about him enough to die for him. Quill had now lost all his parents, but at the same time, Yondu's death staring him in the face forced him to confront loss head on. Allowing him to eulogize, remember Yondu warts and all, resolve to be a better dad for his baby groot, and a better lover for Gamora.
And thus his confrontation with Thanos on Knowhere represents what should've been his final step forward. He'd learned to live with grief of family loss before, but could he finally stand to lose that last someone he felt so close to? Could he live up to her Last Request to kill her, and accept a loss that was intentional and in his control? In the end, the Dying Declaration of Love was said by both of them, and despite the hesitation, Peter did try to follow through for her sake. To finally accept that he couldn't hold onto all the people in his life, and thus kill her for the sake of the universe. Only...he failed. Thanos took his attempted sacrifice away from him, leaving the pain of making that choice, but not the effect of the choice.
Which then leads to his most...memorable moment on Titan: his Tragic Mistake. He follows his girlfriend's sister to Thanos's planet, not just with a goal to kill him for the suffering he's created, but also to rescue Gamora like any hero would. His plan to capture Thanos works, his team and "The Avengers" have him restrained, and since they're about to win, he then demands to know where Gamora is. Only...it's too late. As Nebula figures out, the only plausible explanation was that she was killed while Thanos was getting the Soul stone. Thanos even unintentionally rubs salt into the wound saying "I had to". Heartbroken, outraged, and crushed, Peter attacks, causing Mantis to lose her grip, leading to Thanos getting his faculties back, and eventually, Thanos's escape with the time stone. As the Russo's pointed out in their commentary, "It's at that moment when you realize all hope is lost." Quill failed to make his final leap to maturity, and in the end, he even died for it.
Tony Stark, the genius playboy philanthropist, has always tried to think 2 steps ahead of what's to come. Improving his suits, trying to take responsibility for the harm he caused to the world, looking for different ways to handle his fears. In many ways, he's the most developed character of the MCU. And in some ways, Thanos was partially responsible for much of that development Phase 2 onward.
Because as Tony mentions in the movie, the attack on New York defined Tony Stark going forward. Causing him to become a Shell-Shocked Veteran, and all his actions following trying to be ready for the next big invasion Earth might face. This was bolstered by Wanda using her Mind Stone powers on him, and giving him his worst nightmare: a vision of the Avengers defeated, and another invasion coming to earth uncontested. This lead to him trying to create Ultron to protect the world, only for it all to blow up in his face.
So again, Tony tried to course correct, using his trusted AI J.A.R.V.I.S. as the basis for his next attempt: Vision. This time it seemed to go well, creating a noble, worthy android who could help stop Ultron, and maybe be the deterrent Stark wanted to protect earth. Give him the justification to finally just step aside and rest with Pepper. And then the Sokovia Accords came along. Tony, still wracked with guilt, and hoping to keep the team together so they'd be ready for that invasion, tries to play along after having his ambitions continually ground down, but due to friction, ego and clashing ideals, it all falls apart. Vision makes a mistake, by accidentally hitting his best friend, showing he's not as flawless as he thought. Captain America forces the issue of Bucky killing his family, fracturing trust, and leaving his team seemingly broken. He then tries to make up for it with 2 final attempts. His mentorship of Spider-Man, seeking to get the teen ready to take over as a hero from him down the line, and finally, creating a nano-tech suit. Not for missions though, but as a promise to Pepper. Only as safety to keep their future secure.
And then Dr. Strange and Bruce Banner arrive, telling him about Thanos, and how he's coming for the stones on earth. And Tony realizes that this is exactly what he's been trying to prepare for. The 2nd invasion, and worse still, by the same force that caused the first one. To that end, when the Black Order captures Strange, he goes after them, ultimately forced by pride and fear to finally confront The Mad Titan face to face. To put that genius to use and knock out the foe he'd been preparing for the last 6 years, and that ultimately, even his love for Pepper can't keep him grounded to do this task. (as a more positive reflection of Thanos later on)
He goes forth to do this with his protegee, a team of space heroes, and a master of the mystic arts, but despite their best efforts, in the end, he stands alone against Thanos. Staring his fears in the face, using every bit of skill and tricks he can think of to crush "his curse" once and for all. And despite drawing blood briefly, he fails to do lasting damage. In the end, he's forced to watch as Thanos leaves for earth, clinging to a thin hope that the team he helped assemble will stop him, only to then watch as everyone around him turns to dust. From the guardians, to his rival, and worst of all, Spider-Man dies in his arms. Tony is forced to watch as a young man he wanted to protect and train, a faithful fan whom he had developed a father/son bond he'd always wanted, disappear to ash. He checks himself to see if he's next, but Tony's still solid. After all his preparation, development, hope, and preventative actions, the vision he saw has come to past. His worst nightmare is now reality, and as we pan away from the Iron Man, all he can do is hold himself, shocked and crushed in spirit.
Rocket: And what if you're wrong?
Thor: [mirthless chuckle] Well, if I'm wrong, then what more could I lose?
The creators of Infinity War wanted to set up Thor as a Plot Parallel to Thanos. Making the audience believe that he would be the true killer of the Mad Titan. And it begins by tearing Thor down to his barest. Perhaps even more so than Thor: Ragnarok did.
Thor's journey in looking for information on the Infinity Stones seemed to give him a lighter, more jovial attitude compared with his first two outings, but when Thanos came for his brother, he very nearly broke Thor's spirit. Leading to the death of half his people, his best friend heroically getting his closer friend away from death, and his brother being strangled to death in front of him. Picked up by the Guardians, he resolves to take on one final mission to take down Thanos again, teaming us with "rabbit" and "tree" to go build a new weapon in Nidavilir, as he realized that despite his inbuilt powers he learned in Ragnarok, he needs more now after his first fight with Thanos.
In route, Rocket drills a bit at Thor's mental state, and seeing all the losses he's suffered, asks if he could really handle going at this again. And it's here we see that Thor is using certainty in his own abilities to cover up all the pain he has. As the quote above mentions, Thor sees that he's made it so far for so long, that he believes that Fate itself will see him through to the end, and give him his Revenge. And that ultimately, he's not sure if there's anything more that could ruin him besides death.
Reaching Nidivalir, he strains all his strength and endurance, working with Rocket, Groot and Eetri the Dwarf King to finally forge himself a new ax: Stormbreaker. Tanking the heat of a star to ensure he can get his weapon. And in the end he does, and having willed himself so far, he bifrosts straight to Wakanda with the guardians, rescues his fellow avengers, and begins to break down the battlefield one lightning strike at a time.
Finally, since Thor wasn't given a transceiver like the other heroes before the battle in Wakanda, he's the last on the scene when Thanos rips the Mind Stone out of Vision's head; the last chance that heroics have to stop Thanos before his evil deed.
And at first, it seems that Thor will do just that. Stormbreaker lives up to its boast as a "Thanos killing kind" of weapon, powering through the blast of power, and finally....embedding in Thanos's chest. Thor than lands, and tries to give a Bond One-Liner, proclaiming, "I told you, you'd die for that!" He then shoves the ax deeper, relishing in Thanos's pain, serving his revenge coldly and harshly.
But it's also here that he makes a Tragic Mistake just as awful as what Peter Quill made: by wanting to relish in his victory, Thor fails to remove Thanos's faculties, and in doing so, leaves THANOS with the opening he needs. Right in front of his eyes, Thanos responds, "You should have gone for the head!" and then, snaps his fingers. And in doing so, wiped out half the universe. After this, Thanos manages to escape, and Thor is left with the bitter understanding that he failed. All that determination, drive and power, and in the end his own ego and anger caused him to fail his team, his people, and the memories of his family. Thor has now truly failed to succeed, and is broken with him losing the last thing he could: his facade of confidence.