There are, of course, people who can legally kill another person - soldiers, drone pilots, armed police etc etc. When, in the course of their duties, they do kill someone, how do they disassociate themselves from it? The dead individual, a criminal, a terrorist, whatever, still was another human. Do the legal killers sit down at home at the end of the day and think about the subjects "other" life?
Could you kill someone? How would it sit in your mind?
( We no longer have a death penalty, but how did someone like Albert Pierrepoint square things with having killed literally hundreds of people?)