Right and Wrong

I believe killing people is wrong no matter what; even if they are some unholy union of Hitler, Ted Bundy, and Jeffery Dahmer. However, I believe it is more wrong to allow certain people to live if you can help it; like the afor mentioned people. That doesn't make killing them right, it is still wrong, but it does need to be done. I don't want to kill people but there are some people who I wouldn't mind seeing dead. At what point do you give up, grab a rifle, and solve the problem yourself.