Are there any situations in which you'd be willing to kill another human being that do not directly involve protection or retribution for yourself or others?
Yes. If they willingly participated in a lifestyle that wagered their life against another I would be comfortable with wagering my own against theirs. If I was confident I could do it cleanly without being sloppy I'd be willing to gamble. Gunslinger.
Heck yeah, Let God sort'em out
Never. The guilt would consume me, even without combining with the guilt I already feel.
I could without remorse. I know I'm a terrible person. I should have been a sniper.
I can't think of any, and I would not include "retribution" anyway. Just defense, if it were necessary.