I have not taken another's life but my father had multiple times during war in a close and personal way. Before he died he shared how it changed him.
First it made him agressive, he called it being at a base level like an animal. He said he was changed permanently but had two choices to make: 1. bottle it and remain agressive and probably self-destruct or 2. serve others and use his strength to be compassionate. He chose #2 and gave to community, to people, of himself. He was caring, gentle, kind and also strong. He died happy.
But there was a "switch" that he had. If he ever sensed his family or his person was being threatened you saw something change. In his body language, stance, his face, his eyes. He was containing a triggered violence within him and he was very capable physically so you took it seriously. It was his demon and he conquered it, but it was also on-call.
"One resists the invasion of armies; one does not resist the invasion of ideas" Victor Hugo