Except you don't know, you cannot know, what someone is going to do in the future. You are making the big (and in my opinion unacceptable) assumption about what someone killing the Quaggoth will do afterward. Essentially you're preemptively killing people because you think they may later come after you in some situation. Maybe you're right, maybe you're wrong, but part of being a rager, and especially being a drillmaster, is not having the right or choice to go kill people because you think later they might come after you. Even if they do help others raid, if they aren't enemies you aren't supposed to kill them until they are marked hunted, usually for having raided many times.

By your own words you've described doing exactly what a rager, and again in particular a drillmaster, should not do. That's what I have issue with.