Contains ending spoilers
In the last episodes, we are shown the evil Homunculus facing off with (the) God (of his world). This is a standard stand-off between God and Satan, with God being wise and forgiving, yet ruthless when it comes to punishing, and Satan who is unaware (or incapable, or not wanting to admit) of his sins, being punished for all he's put the humans through.
The Homunculus appears in the human world as the result of human action, who most likely wanted to get to the "Truth", the forbidden tree (outright drawn on the gate), and instead got a "fake messiah", an entity already looking rather Chaotic Evil. At his beginning however, Homunculus may have been harmless, and just "a small fish" like his sin, Envy, turns into. He may not even be responsible for getting to the human world, although it is possible he may have wished it and may have influenced events in the human world to arrive there. Still being Affably Evil at the time, he tries to understand humans, only to see their greed and their interest in him, and to be risen by them to the status of a superior creature. He probably laughs at this at first, then it goes to his head. "Hey, why can't I be what they think I am? I deserve it. I deserve a body." So in a sense, humans corrupt the devil, and the devil only uses their own ruthlessness and lack of scruples to further its agenda. As we see, Homunculus is always stuck in a flask, doesn't have any magical or persuasive powers over the humans, but the humans dig themselves their hole.
King Xerxes doesn't want to die and is ready to make a deal with the Devil and do his bidding, if it means living forever (which, incidentally, he got his wish, just not how he expected). Only one word from the outerworldy visitor is needed for him to spill rivers of blood for a faint hope. At this time, Homunculus is amused by the humans' stupidity and starts to only see them as a means to his goal, as simple object to his superior intellect. He makes the mistake to confuse human stupidity with the effect of his own smartness and power.
But Homunculus has not turned into the Devil yet. He's still just a Psychopathic Manchild combined with Insufferable Genius as far as his knowledge are concerned. And in a way, he respects his father figure, Hohenheim, who is like a Doctor Frankenstein to him, and will act the same way, abandoning his protegee when he acts "monstrously". So he decides to model his body according to Hohenheim's, his long time companion (who he possibly even called "brother" in the second anime), and not by the decadent king, or some young servants around (Orochimaru, I'm looking at you).
The meaning/circumstances of Wrath's death in the manga and second series
Contains ending spoilers
Wrath ends the series as something of a Karma Houdini, with the heroes giving the official story that the evil members of the State Military were traitors to Bradley, and that he died a martyr's death in the battle for Central. There is some poetic justice in a man devoted to war having his name used to spread peace, but it might still be questioned whether it was fair for him to die composed and happy with his life. However, it really couldn't be any other way. Bradley/Wrath is all about the idea that only the strong can survive, and he scoffs at the idea that anyone would sacrifice their life for person (or really, even give value to the lives of other people). Related to these too, he doesn't really seem to understand or value acting with an honorable motive.
Thus, the problem with having Lan Fan, Greedling, or even Mustang utterly curbstomp him is that it would validate his philosophy. He doesn't really have a problem with being beaten by someone stronger than him (being a social darwinist), it's more like he thinks it's impossible for someone better than him to succeed. Further, if someone killed him out of revenge, that would support his cynical reading of humanity.
While it's true that Greedling does deal out some significant damage to him and he does die pretty messily at Scar's hands, both happen in a way that challenge's Wrath's beliefs. Greedling is only able to harm him because of Fu and Buccaneer's Heroic Sacrifice (something Wrath would see as a weak/hopeless act), and despite ample motivation (and a history of being obsessed with revenge), Scar doesn't fight for vengeance- he only does it because he needs to get through him in order to help the others.
This might also play into the ambiguity of Wrath's death. There's some implication that very deep down, he does respect human values, and is a Death Seeker. From this reading, he dies happy because at some level, he didn't want the homunculi to win. On the other hand, it's quite possible that he's a case of Evil Cannot Comprehend Good to the end and dies happy thinking his philosophy was validated, even if the audience knows it wasn't.
How the definition of wrath, not actual combat skill, decides the outcome of Wrath's fights
Contains spoilers for most of the series
The biblical sin of wrath has less to do with a taste for violence and more to do with revenge — it's the opposite of turning the other cheek, so to speak, and instead advocates for striking back at whoever threw the first punch. On his own, Wrath is not a violent person, and all of his kills were done in response to someone else trying to kill him first. Looking closer, many of the fights that Wrath has either failed in, lost, or suffered an injury from are structured so that his wrath was not justified or the violence inflicted on him was absent of wrath. To elaborate:
- The numerous times when a Red Shirt attempted to kill Wrath always resulted in him returning the favor. Greed's men at the Devil's Nest and the Briggs Army during the siege at Central HQ are the biggest examples. Wrath even retaliated against characters important to Father's conspiracy — he jokingly cut the top off of Ed's spear during his exam, and Mustang's attempt to burn him resulted in some nasty Impaled Palm that was pretty unnecessary considering Pride could have pinned Mustang himself.
- The first instance where Wrath fails in his violence was when he attacked Lan Fan and Ling. Lan Fan survived his attack because she was acting in self-defense; Wrath struck first, and despite dialogue changes in the anime, the original manga says that she held up a kunai to cushion the blow and gives her a line stating that she had no quarrel with Wrath. Ling's swordplay, while certainly the best on the heroes' side, was also done in self-defense. His strategy was constantly on escaping and protecting Lan Fan, so one could argue that Wrath's violence was unjustified in terms of revenge and led to his failure.
- Greed the Second's attack on Wrath in his own home is an interesting case. Greed is certainly wrathful, but he was acting in retaliation for Wrath's massacre at the Devil's Nest, where his goal was ostensibly to capture Greed and his soldiers were meant to carry out the violence. The context of the fight forced Wrath to act in defense of his family instead of in vengeance, but since Greed's attack wasn't self-defense or free of wrath, the fight was a draw.
- The bombing of Wrath's train, while framed as a No One Could Survive That! assassination attempt that unrelated characters treat as such, is never treated as an act of murder by Grumman or the engineers who carried it out. One could argue that they knew it would only slow down Wrath, while Olivier and the rest of the soldiers who'd taken Central had become overconfident and thought it would have removed him from play for good.
- Wrath's battle at the gate:
- As stated above, the slaughter of the Briggs troops who fired on Wrath was a given. Buccaneer survives Wrath's retaliation because he, as the Briggs captain, was trying to avenge his troops, and he succeeds in disarming Wrath because it was a nonviolent action. He still initially fails in his attempts at violence because they weren't free of wrath.
- Falman never gets attacked by Wrath because he never attacks, but if Greed hadn't shown up, his act of raising his gun might've changed that. In the resulting fight, Greed was roughly on par with Wrath for similar reasons as before; neither's violence was or wasn't quite justified.
- Fu's vengeance was entirely justified and even done on behalf of someone else, but he wasn't acting in self-defense and he eventually lost. However, Buccaneer's sneak attack succeeded because it was free of wrath — it was an act of violence that benefitted the heroes immensely, but Buccaneer and Fu were beyond help and they would not have lived to see its effects. Additionally, as a sneak attack carried out by two dying humans, it undermined Wrath's belief in survival of the fittest and fighting enemies head-on.
- Ling's violence wasn't entirely justified or in self-defense, but his actions of disabling Wrath's Ultimate Eye, shattering his sword, and attempting to throw him in the moat were geared toward removing him from play and giving Ling a chance to offer his Stone as an alchemic aid. The soldier who fired on Wrath received a revenge wound for his trouble, but it was the only way he could rescue Lan Fan and Greed, so attacking Bradley was justified enough to avoid a fatal retaliation.
- Before Wrath's last fight, he invited a handful of characters to attack him, but none of them had any good reason to take him up on it. Instead, he ended up fighting Scar, who had all the right reasons to win the fight while Wrath had none.
- Scar had grown out of his own vengeance directed at alchemy and Amestrians. Additionally, he wasn't fighting to kill; he needed to get past Wrath and activate the reverse transmutation circle. His turnabout moment consisted of accepting and utilizing his brother's research, which he had condemned as blasphemous in his earliest chronological scene.
- Meanwhile, Wrath had ignored his role in protecting the center and keeping the heroes from reaching Father just for the fun of one last fight. During it, he constantly tried to awaken vengeance and wrath within Scar multiple times without success.
- After falling in combat, Wrath suffers his final defeat at Lan Fan's hands, though not in combat. He tries one last time to awaken wrath in another person, but Lan Fan ignores his invitation to avenge her grandfather. Instead, with a mixture of pity and contempt, she asks him if he was ever capable of love. As soon as Wrath responds by talking about how much he valued his wife and how humans made his life worth living, he starts to age and his death speeds up considerably — and when he finally dies, he does so without a trace of the wrath he was named for, conquered in combat without resorting to rage and speaking to one of his worst victims about the love in his life.
- Everyone who lost to Wrath struck at him without provocation, thus justifying revenge.
- Everyone who fought Wrath to a draw or survived otherwise acted with vengeance of their own. Alternatively, they acted in self-defense or with a lack of Killing Intent that made his violence less justified.
- Everyone who succeeded in physically harming Wrath to any extent had some form of selflessness in motivating their violence.
- The person who killed Wrath, Scar, fought with the utmost selflessness and only used enough lethality to give himself a clear shot at doing his duty. He'd also discarded the wrath that had previously defined him. Meanwhile, Wrath had neglected his duty to protect the center and decided to start one last unjustified fight, thus casting aside a good reason for violence in favor of attacking someone who had never attacked him before.
Scar's deep psycheAn excellent analysis of Scar's motivations behind his actions is found here.
- Scar didn't explain to Winry that his murder of her parents was accidental because he didn't want forgiveness for his crimes.
- Scar's goals were to die at the hands of those he wronged (particularly Winry) and to take as many with him as he can.