Some people, out of necessity, have taken other lives to protect lives. The guilt sometimes haunt these people.
But maybe that shows a person who is still sensitive to the loss of life, and wishing it could not be lost -- even that of an enemy.
The New York PD dropped their "Stakeout Squad" Program, not b/c it didn't work, but b/c they were losing too many good cops, after they killed people. Killing affects you, even when the people you kill are human filth...