feotakahari

 Bad guy A is about to brutally murder innocent victim B. You intervene to fight A and save B. How prepared were the developers for the idea that someone might try this?

Dishonored 2: you get an achievement for your heroism.

Shadow Tactics: B hides inside a nearby building. In gameplay terms, it's the same as if they'd died. (Neverwinter Nights does this too, but you at least get a boost to the ol' karma meter.)

Splinter Cell: you interrupt the scene. A will keep hunting for you and won't go after B again. B will just sit there forever, because he wasn't programmed under the assumption he would ever have to acknowledge your existence.

Thief 3: A and B are both coded as your enemies, so they'll work together to beat you up.