Why did they kill Osama!?

Yes i believe he was guilty of what i can't say because i'm not educated enough. But why did they kill him without a fair trial? That was wrong. I don't care what you guys say about 9/11 what United Sates did was messed up. They should have took him into custody period, he was not armed they just said there was no security at his compound & his daughter was his daughter. I feel bad for the victims of 9/11 too. But you don't fight crime with crime !