The worst part? In Portal 2, GLaDOS says the companion cubes are sentient. This is right after she vaporizes two or three cubes in a single test chamber.
In Test Chamber 7 of the sequel, you run into another Companion Cube. At the end of the chamber, GLaDOS tells you that while the Emancipation Grill would normally eliminate anything you try to take with you, this one is broken, so don't take anything. So you go to get it back, take it down to the elevator and...
The Companion Cube poetry behind the walls of level 17. Poor Rattman...
The first time you knock over a turret, only to have it say, in a melancholy voice, "I don't hate you..."
If you use a turret as a shield while a different one is firing at you, your makeshift shield screams, "W-wait! No! ...It's me!"
The turret voices have been updated in Portal 2, with a heartrending "Nooooooo!" if you fling them into space. Thankfully, the turrets forgive you in the end and sing you a song.
The good turrets being thrown into the Redemption Center after correctly responding to the template.
Turret: But...why? I did everything you asked! *CRUNCH*
The Curiosity Core is a childish, naive little AI that asks questions about anything it sees and cheerfully greets you once it recognizes you, all in a cutesy voice that sounds like the turrets but absent the creepiness. It's also the first core you incinerate. Its scream is also pretty horrible to listen to.
'"Look, we're both stuck in this place. I'll use lasers to inscribe a line down the centre of the facility! And one half will be where you live, and I'll live in the other half. We won't have to try to kill each other, or even talk if we don't feel like it!"' [GLaDOS] actually sounded a bit freaked out here...or as much as a computer/robot/AI thing could.
In the sequel its revealed early on that GLaDOS spent the time since the first game being forced to relive Chell killing her over and over. And that's not even counting the crap she goes through later. Combined with the fact that Valve went out of their way to make her more sympathetic as the game goes on... Fortunately, she gets a relatively happy ending.
Finding out that Chell gets dragged back into the lab again was depressing enough, but hearing that in Portal 2 she wakes up centuries in the future, Jesus, there is literally nothing left for her outside. The world outside the lab is a completely different place by now, and she can't even have the Tragic Dream of going back to a normal life anymore. She doesn't have anything worth fighting for anymore besides her own life.
Not only that, but as revealed in the comic, Doug Rattmann attempted to save Chell from getting dragged back into the labs, ignoring his spirit Companion Cube telling him to run, insisting that he had to try to save her. Seeing that the Party Escort Bot had put her in an offline cryo-stasis chamber, Chell would have died if it hadn't been for Doug activating the chamber, even though he knew that he couldn't set an automatic wake-up time. Without the intervention of Wheatley, she would never have woken up.Tear Jerker for Doug and Chell indeed.
Rattmann's dialogue during this part is pretty sad as well.
Rattmann: So it's the long sleep...or the long sleep. And I don't know which is worse. Forgive me.
Towards the end of Lab Rat, the Companion Cube is asking Doug how he knew that Chell would be able to do it, how he knew she could get the two of them out. He gives a perfect non-answer, saying that he just had a hunch. It's not until after he dies that we learn why he chose her.
Proctor's Note: Test subject is abnormally stubborn. She never gives up. Ever. Rejected, do not test.
The end of Chapter 5, if you think about it. You're so close to achieving your goal, Wheatley is getting you out at long last, only for the triumphant music to suddenly take a dark turn... and you watch as everything starts to go terribly wrong and Wheatley, your one friend starts to lose it, mad with power and suddenly turning on you, claiming your work was his. The worst bit? When the music swells again, you can hear nearly the exact same victorious music from merely a MINUTE ago, when you had seemingly won, but now it serves as a Dark Reprise of what you almost had together. Why, Wheatley? Why?...
Even after hearing all the hilarious craziness of Cave Johnson in the sequel, you'll be sad by the end. Cave Johnson made some amazing things and had absolutely no morals whatsoever, but...everything he created, he did by accident. He wanted to make diet pudding; he made repulsion gel instead. He wanted to make a fuel injection system de-icer; he got a psychotic AI instead. This is all very funny...until then you find out he's dying from exposure to all the crazy things he's experimented on, and despite the amazing devices his company has created, by the end they can't even afford to pay the homeless and unemployed a few bucks to act as test subjects. Even his plan to save himself by uploading himself to a computer never works out...for him, anyway.
The real moment is just at the end of his rant. Even though it sounds mostly in GLaDOS' usual monotone, something about this moment...
GLaDOS: Goodbye, sir.
What makes the above (including the farewell) even more sobering is when you stop and actually realize that these recordings are all that's left of Cave Johnson, and that they'll almost certainly never be heard again. They, along with his image and the parts of the facility he oversaw himself, will now sit underneath Aperture Laboratories forgotten, in silence, forever.
"Haec omnia transeunt" All things pass away
It's implied in the Co-op mode, however, that GLaDOS eventually got to link herself with the old facility's power grid, which means that she, along with her new batch of test subjects, hasn't seen and heard the last of Cave Johnson.
His undeniably awesome combustible lemon rant is quite sad when visualizing what he must have looked like when recorded it. A near penniless man overlooking his empire as it stands on its last legs while slowly dying a painful death, hunched over a tape deck after he just downed another round of pain pills, screaming his frustration at life while tears stream down his face, most likely breaking down into sobbing after the tape stops recording. Even sadder is that his misery is almost entirely his fault to begin with, and his screaming rant is likely an attempt to deflect his own personal responsibility.
The recording you hear before the Conversion Gel chamber. "Welcome to the Enrichment Center...*cough*" At this point, with just that one line of dialogue, you already know everything about Cave's situation. He's broke. He's sick. He's completely demoralized. The enthusiasm of a once magnificent entrepreneur and one of the greatest minds of a generation, gone. All that remains is science, and so, almost as soulless as the robots installed to replace his management, Cave greets you with a simple line...and yet the way he says it, all the subtext of a defeated man...
For that matter, the obvious crash Aperture goes through as told by Cave's recordings and portraits. First batch, he proudly congratulates you (presuming you are an Olympian or an athlete) on helping to contribute to science, and his portrait is a smiling young man. Second batch, he's muttering under his breath about having to recruit hobos and homeless people on the promise of sixty dollars, and his portrait is an older man going bald but still looking proud. The third batch has the defeated and sick Cave saying employee testing is now mandatory, and his portrait is a bitter and haggard old man.
Wheatley floating through space in the ending of Portal 2. Especially because he's genuinely sorry for what he did, but in all likelihood, he'll never be able to make it up to Chell.
Even moreso when you remember that he honestly couldn't control himself when he was attached to the body. The fact that he remembers it means he knows he did it, and he probably didn't want to hurt Chell at all.
Another part is when Wheatley screams "GRAB ME, GRAB ME!" Given just shortly before in the boss dialogue, he revealed how hurt he was that you never caught him, it just twisted the knife.
For a bit of horror mixed in, remember that this IS the Half-Life universe, and there are very likely to be mind-breakingly horrible things in space.
At the end of Portal 2, when GLaDOS says she's letting Chell go and deleting Caroline. She could've killed Chell and deleted Caroline easily while you were knocked out, but she didn't. It's implied by her actions and "Want You Gone" that she lied about deleting Caroline to intimidate Chell into never coming back, probably because GLaDOS is worried she'll go insane again.
In typical GLaDOS fashion, she contradicts herself repeatedly in the ending song, first admitting again that you were her "only friend", then denying that she was talking to you, then erasing the words "Maybe I'll stop feeling so bad" from the screen even though she still sings them. You're watching her struggle with complex and conflicting emotions towards someone who's become so important to her, first as an enemy and then as an ally and then as a friend, and she is struggling with it even as she desperately pushes you away and denies that "She needs anyone".
What's more, in another tear-jerking occasion at the end and also possibly a contradiction, the turrets singing the "Cara Mia" opera at the end. Translated, the lyrics are a final farewell to Chell as she ascends to the surface, possibly organized by the Caroline part of GLaDOS. Personally, it was perhaps the most emotionally affecting part of the whole ending (and not because of the beautiful soprano).
Of course, as she herself states "Killing you hasn't worked out all that well in the past."
This is after she admits that you weren't her greatest enemy, but her best friend. The ensuing Mood Whiplash is what makes it so sad.
Note too that a few scenes before, GLaDOS mentions that she's hearing a conscience and it's her own voice. Where as before, she may have been conflicted between a handful of negative emotions, now GLaDOS has to deal with more positive ones that outright contradict everything else she's thinking. In short, she now has a Wheatley she can't ever get rid of and one that's much more powerful than the actual Wheatley.
Right before the above example, when Chell wakes up after the complete and utter Lunacy:
GLaDOS: (genuinely) Oh, thank God! You're all right!
When you wander into the Bring Your Daughter to Work Day Science Fair in Portal 2. There are names on all of the projects, and on the very last project, in small grade-schooler scrawl, is written 'Chell'. This has led to theries that she was a daughter of scientists working there, and they were most likely killed by GLaDOS.
Playing Chapter 9 right after 5 is absolutely heartwrenching. Watching what was basically your best friend go mad as the rush of power reinforces every negative emotion, then having to take down said friend while he raises some concertingly good points (You DID drop him and you DID fail to mention GLaDOS would be rather pissed if she ever woke up) is an emotional sucker punch.
Wheatley'sVillainous Breakdown during the final boss fight. His "The Reason You Suck" Speech is less like an villain's evil gloating and more like the ranting of a heartbroken child who doesn't understand why everyone has turned against him, or maybe that of a scorned lover who hates you for choosing someone else. As he starts to lose it more and more, he accuses you of taking advantage of him all along as a means to escape, then abandoning him when he needed you most so that you could run off with your "best friend over there." He even brings up the fact that you didn't catch him when he dislodged from his management rail, which he honestly believed would have killed him, and hysterically accuses you of missing on purpose because you wanted to get rid of him. (This is made much, much worse by the fact that even if you honest-to-god tried to catch him, it's impossible to do so.) What makes it even worse is that, if you listen closely to his voice during the rant, he sounds an awful lot like he's crying.
And it's enhanced by the music- if you listen between cores, you can hear a remixed version of 'I AM NOT A MORON' inside the adrenaline-pumping Boss Battle Music- his victory music is playing. Aperture is literally crashing down around him, and he's about to lose his power- what crueler way to accentuate that than a Dark Reprise of his victory tune?
Although still sad, some of this is tempered a bit by the fact that the only reason you 'abandoned' him was that he went insane, trapped you in an elevator and punched you down an elevator shaft.
Granted, if you stand there long enough during the final boss fight, he starts ranting about how much he loathes you, and that everything could have been perfect if not for you. Poor Chell.
During the final boss fight, the Aperture Science Handheld Portal Device is the first thing to get sucked out into space. Goodbye, old buddy...You served us well.
Leaving the Oracle Turret behind. Twice.
When picking up the Oracle Turret for the first time, she just sounds so relieved and sad and exhausted in those two words: "Thank you..."
Exile Vilify just brings tears to eyes. Especially after reading the Lab Rat comic, and learning about Rattman's past.
The song was apparently originally used for an easter egg ending, presumably one with a not so happy feel.
You think the song's sad enough on it's own, right? Well, guess what Someone made a stop-motion music video of the song with Rattman and Chell. It makes the song go from tearjerking to outright depressing. You can watch it here.
Here's another music video, made specifically for a contest, with Wheatley singing it. Sounds ridiculous, doesn't it? Well, that is until you realize that he's singing it in space...
Is Caroline's consciousness really stuck inside GLaDOS, alone and unloved, for eternity? That's pretty heartbreaking.
GLaDOS sounds so sad and alone when singing 'Want you Gone'. The strongest part of the song is either how she tries to retract the lines "maybe I'll stop feeling so bad", but still stays them, or the part when GlaDOS says "I don't need anyone now." The way she sings it is really sad: a bit deeper, and her voice trembles as much as it's possible for a robot. It sounds as she's forcing herself to say it and/or is in the verge of crying.
In Co-Op mode, one of the voice-overs GLaDOS is her saying you both have gotten very close, and she's known one other team like you. One was "an imbecile" she had to destroy, the other she doesn't elaborate on. Obviously, this is referencing Wheatley and Chell, and how they did have a friendship between each other as they tried to escape the facility, before Wheatley goes mad with power. Thanks for depressing me with the reminder that Wheatley is stuck in space, unable to apologize for what he did, game!
Most of GlaDOS' comments in Co-op are sad when you realize she's saying things she told Chell in single player Because she might miss her after all. Like the deer, the weight thing, ect. Plus her reference about how human subjects are better doesn't help... She says it in a cruel way, but that's just how GlaDOS is. Tsundere much?
In the final Co-Op level, when you find the vault containing human subjects you see a reprogrammed turret which screamed "I need to protect the humans!" The turret is quickly killed off by GlaDOS just so she could deploy blue gel (and possibly silence the turret).
The way GlaDOS just casually crushes Wheatley after waking up.
The way GlaDOS screams in pain while you replace her with Wheatley may result in this, with a dash of What the Hell, Hero? since it's entirely your fault.
Some of the alternate Cave Johnsons:
The Space Warden Cave Johnson gets shived by criminals during a power blackout. He is actually in the process of dying when he addresses you.
CaveDOS, who, unlike GlaDOS, begins to realize that Brain Uploading himself was a terrible idea. He's still hilarious though.
(on Hercules) "[He] defeated all of Earth's monsters so humanity would be safe, and then went to Olympus for his trouble. ...Dammit. Death was my monster. And I killed it... Where's my Olympus?"
PotatOS Lament, the menu theme that plays while you're in the old Aperture Science testing spheres is a sad piece of music on it's own, but it's made even sadder due to who's singing it: GLaDOS herself, after being usurped by Wheatley, turned into a potato, and tossed down an elevator shaft into the depths of Aperture. The sheer emotion in her voice is enough to shed tears.
Fan theories have suggested that it's actually Caroline singing and not GLaDOS . This actually makes things worse: Caroline had already gone through the procedure that would transform her into GLaDOS and went crazy, but at least she had control over the Aperture Science facility as a consolation. Now she's been stripped of even that, and thrown into the depths of Aperture Science, where (had Chell not found her) she would have laid. Forever.
This is definitely YMMV given people's opinions, but every single time Wheatley says "I! AM! NOT! A MORON!" can be a Tear Jerker, because he sounds not only offended, but heartbroken that GLaDOS and even worse, his best friend, thinks he's stupid, and the fact tha GLaDOS constantly asserts he is one makes all of his plotting look like someone desperately Feigning Intelligence. The idea that Wheatley tries to kill someone just so they'll think he's smart is eerily similar to the behavior of neglected children who do terrible things so their parents will be proud or even just notice them.