It does start with a dead body, after all, and a scramble to identify the culprit before more former superheroes can fall victim. It’s the sort of story that’s expected to have a surprise ending, or it hasn’t done its job.
The inevitable villain reveal part of the twist is a perfectly satisfying one. It turns out that one of the ex heroes, Ozymandias (a.k.a Adrian Veidt), has a massive god complex and is trying to prevent the other heroes from interfering with his plan to unite the world and avert a nuclear apocalypse by staging an alien invasion (and killing millions of people in order to sell the act).
That much makes the twist satisfying, but not exceptional. The extra push that puts it on this list happens here:
Marge: It’s an ending. That’s enough.