Humans are not quite as psychologically flimsy as we like to paint them. I personally believe the foundation of your logic- that anyone who is not mentally ill will suffer lifelong depression and unhappiness after killing someone- is unrealistic.

Just think of examples of self defense, or crimes of passion (e.g. the killing of your lover's mistress). Why do we never hear of how all of these people fall into bottomless pits of depression?

Particularly in your description of it "destroying" someone, I wonder if you are thinking of "Crime and Punishment". Remember that was not a crime of passion, but a planned killing expressly chosen to be in cold blood. Killing in cold blood is a different story, but we are not talking about that.