Killing in some cultures is tolerated, even rewarded. In ours, it is not. Personally, I would not think twice to kill someone who was out to hurt me, my family, or anyone I loved. Honestly, I think people like child molesters should be lined up so that we can practice long range shooting for the upcoming hunting season.Unfortunately that will never happen.
Back to my professor. His argument was that killing was not moral. Other cultures who thought it was moral were wrong and that those cultures should be forced to recognize it as being wrong, if it ironically means through war.
My argument is that, morals come all the way down to the individual. You have the freedom to kill, steal, etc. However, you also will have to deal with the effect of your actions. In our culture, that would be imprisonment or death. If someone came and murdered my family in their sleep, I would have no problem finding that person, and killing them the slowest most painful way possible.