So you wouldn't kill in the defense of your loved ones and every single person on the island? As I said earlier in the thread, part of training someone to carry a weapon to be used to defend themselves, loved ones, or innocents, is to have them come to terms with taking another's life. Also to define the limits when they would use potentially deadly force.
"Normal people" who decide to provide for their defense and that of others, are led to come to terms with the necessary boundaries to do that. Some decide they can't do so. Fair enough. Those who move forward are willing to kill if necessary within the boundaries where they have no choice.
"Normal people" facing an ugly enough threat have killed the threat for thousands of years. As I said earlier, the MC had no choice, what sucks is had they left as soon as they knew of the plague he never would have been in the situation.
Also we (not you but several folks here) have been throwing the term assassination around pretty freely. Assassination applies to killing prominent people heads of governments, major figures etc. Killing the inquisitor is murder, but not an assassination.