Well "direst circumstances" is a bit rich. I'm quite sure if the circumstances were dire enough even my dear old mother would kill.
And what are we talking here? Are we saying flip a switch and they die in a remote location of bludgeon them to death with a blunt object? I'd think if it was particularly tough or gory I'd just shut down, can't process it all and be unable to do anything. Happens a lot apparently, when the shit hits the fan they just go catatonic.
But if I was being attacked and I had a gun, yeah, I guess so. There's not much to think about, if it's my life or his my lower brain functions are going to make the decisions for me.
But I don't have a gun because I live in a "peaceful society" (pah!) where guns are de-facto illegal. In fact the law doesn't look to kindly on self defence either, so that's another lay of inhibition. I think I'd freak out to be brutally honest, lack the nerve to do it even if I felt justified.