Fullmetal Alchemist (manga)/Analysis
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 scoff's 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.