Muammar Qaddafi appears to have been murdered. He was captured, perhaps already injured but still able to stand and to speak, and then he was shot to death. That's a war crime and a regular crime.
His murder should not surprise us. He ruled by brutality, and died the same way. But it is worth noting that this is not simply poetic justice. Rather, it reflects a fundamental truth: that the rules of war, and other rules that somewhat restrain the utter ferocity of conflict, depend on mutual adherence for their continued effectiveness.
It might be a good thing if human beings who were regularly subjected to outrageous and unlawful violence characteristically continued to live by the very rules whose violation was wreaking havoc upon them. Whether it would or wouldn't be a good thing depends on your assessments both of strategic calculations of deterrence and your views of the reasons for the binding force of moral rules. But whether or not such conduct would be a good thing, it isn't likely. People don't act that way. We're too quick to anger, too skeptical of each other's intentions, too impatient with enduring suffering. Not everyone, but probably most of us.
If you live by the sword, you will die by the sword -- if your enemy can get his hands on a sword. Qaddafi's did.