Yes, he was a very, very bad guy. Almost everyone who seeks political power is. Any one of his victims would have been fully justified in killing him in self defense. But... the old monster was not a credible threat any more- he was no longer the ruler and was not engaged in an initiation of force at the time he was killed. He was killed in revenge, not in self defense. I also don't see any justice in it.
I think revenge is wrong. Understandable, but wrong. I'm not saying I would never again do anything in revenge, myself, but if I did I would be wrong. I have done a few things in revenge in the past. I was wrong.
When I smeared poison ivy all over the toilet seat, and my wife's vibrator, before I abandoned my house to her and her new boyfriend, I was wrong. If I had it to do over again I hope I would make the right decision this time. I have the benefit of years of libertarianism to guide my actions now.
The rebels, who are now falling all over themselves in the rush to set up another State to abuse them and violate their liberty, were wrong for killing Gadhafi. And they are wrong for allowing a new government to be established. Actions have consequences. Watch what happens next.