Thor kills demons and giants and such when there's no other option and refused to do even that on Earth for years out of respect for modern society's standards. Captain America is a soldier who's killed in battle but absolutely, positively does not kill if there's any possible way to avoid it; there are several stories devoted to hammering home that very point, in fact. Tony's been using a lot of lethal force since Civil War, but before that he generally tried to avoid it. Not as hard as Cap or Thor or most of his fellow Avengers, but he did try most of the time.
The rest of your points don't even make sense. Having an unstable mind does not magically equal murderer. Hank Pym's got a lot of neuroses, but he's been fully in control of his actions since he recovered from his Yellowjacket period when he hit Jan around 30 years ago. The Hulk's got his math-powered ability to avoid killing people, although he does still kill now and then. Plus, the Hulk's barely an Avenger in the first place. He helped the other founders out against Loki and then turned on them in the very next issue.
In any event, your entire post neglects one major fact: A willingness to kill is not equal to being an assassin. X-Force is a team of assassins whose sole purpose is to murder people. The Avengers are not in any way, shape, or form anything like that. Never have been, never should be (although with current writers, who knows what they might be reshaped into one day).
