How does Marvel justify former villains becoming Avengers?

I was pondering this question after reading some of the post-M-Day X-Factor stuff, wherein Quicksilver caused a lot of death and destruction with the Terrigen Mists. I know that Wanda was retconned into having been a victim, similar to the whole Jean Grey/Dark Phoenix thing, but how did/do they explain away things like this? I'm honestly very curious, as I haven't read much Avengers stuff since around AvX and I could never find a real explanation. Do they just not know he is a sociopathic mass-murderer? And what about all the others who can't excuse themselves as being under the control of a Hulk/Void/Life Force/Phoenix entity?