In the Marvel universe, the Avengers serve as the ultimate band of heroes, with the X-Men as a close second - due to the other also serving as the world's wildest boarding school. Being the ultimate band of heroes, however, also means that a lot of difficult situations are handed to them, which should go some way in explaining why such a team of good guys have done such terrible, terrible things.
With a planet and universe under constant threat, as well as a team roster continually trying to murder each other, it's only natural that some past actions have not been completely morally just.
There's something about seeing heroes in situations where they need - or feel the need - to do bad things that is just completely fascinating; some of the most interesting Avengers storylines come from situations where there is no right choice, only different consequences to deal with.
Between secret cults, zombie armies and outright genocide, it would be easy to think that the team stays in business by avenging their own past misdeeds...
10. Letting Children Get Kidnapped
How many heroes does it take to keep track of alien children? Apparently more than you'd think, as several members of the Avengers are tasked with teaching and protecting the Children of The Sun, and yet somehow totally miss them be taken away by Pterodactyls.
Given kidnap by dinosaur is not the most sneaky method possible, it's easy to think that maybe the team weren't giving it their absolute all. The most ironic part is that this happens during one of the many, many conversations had about how precious these aliens are, and how important it is that they are kept safe - a conversation apparently so engrossing that it let the exact thing they were talking about stopping actually happen.
The children are eventually rescued, but there's something distinctly concerning about the heroes tasked with protecting the universe being unable to protect a handful of blue babies.