Other than self defense what would it take for you to be morally ok with killing another human?

Probably when someone wants to be put out of their misery.

Any time there is consent.
Either explicit, as in assisted suicide, or voluntary combat (sport, dual, whatever) .
Or implied, as in someone attacks you, or a soldier of an army that is at war with you. Situations where the person would expect death to be a consequence of their actions.

Defending the life of another also qualifies as a morally just reason for taking a life.

As for vile crimes in which you're exacting revenge, rather than preventing a crime through force (compare someone with a gun to a person's head to one minute after that gunman has killed the person already and is not threatening anyone else actively), I do not believe murder is justified. No matter how vile the crime, how perverse the offender, if there is not an imminent threat, they should be arrested and dealt with through the justice system (which, by the way, has the ideas of revenge and retribution built into it).

Although I don't personally approve of suicide (for 99.9% most of us in the west, I don't think any situation is so terrible that suicide is actually the best course of action; edge cases of mental illness/being in an extreme situation like a Syria or Liberia, you could maybe justify it), I do believe in the right of individuals to partake in it. So although I wouldn't recommend or help someone with a suicide, I don't think they're committing a moral wrong by taking their own life.

And as @Innocentbystander said, consent makes it morally acceptable for those in that situation, although personal qualms would prevent me personally from engaging in that activity.