Don't forget about Wolvie. He's killed more people than Hitler.
#1 I'm gonna need to call in some fact checkers to do the math.
#2 Most of the people Wolverine has killed probably had it coming.
#3 Wolverine can be easily manipulated so they let him run around until they need him again. If they locked him up, he probably won't wanna do other peoples' bidding.
#4 He'd probably kill even more people if they tried to lock him up.
#5 He'd probably kill you for saying that.