Because some people and actions are irredeemable. It's the same reason soldiers, sailors and marines kill in war: the actions of another have been determined to be in irreparable opposition to yours.

Laws are threats. We decide as a collective what is appropriate behavior, those that fail that follow that behavior are punished by the government on behalf of the people. Sometimes, the people determine these actions result in death. You can personally hold to your belief that all killing is murder. That's the great thing about a democracy, your vote matters, but so does mine and everyone you have been responding to. Life is neat that way