Through ideological consistency. I'll use dogs as an example. If you walk up and kick a dog in the ribs, you're either a psychopath or likely to become one as you continue that behavior. If the dog runs up and attacks you, you shouldnt feel remorse for kicking it in the ribs. It's self defense. If you see a rabid dog roaming the neighborhood, you wouldn't feel bad about shooting it given the clear threat and incurable nature of rabies. That being said, the comparison between some malevolent humans and rabid dogs will be very fact specific and circumstantial. However the point still stands - if a living entity is malicious, dangerous, and cannot be persuaded to cease its violence...you are *wrong* to regard yourself as unethical in defending yourself. Hold yourself to high moral principles and dont waffle. Evil is evil and not to be pitied.