Recently in Japan, we had a trial for this SK who killed numbers of small children's. In the trial she said something along the lines of "I feel sorry for those I killed. But I don't understand why we can't kill."

This got me thinking. I know killing is bad. But other then "Because the law said so" and "Because it said so on the Moses stone tablet" I couldn't think of valid reason why killing is bad.

Could anyone inspire me please?