In Apocalypse World, there is no move that allows you to say, "I have you helpless now; I can keep doing harm to you until you die, and there is nothing you can do about it." In stories, furthermore, when Fifi is lying there in a drugged stupor, and Rouge looks at him with murderous intent and then gets ready to stab him in the eye with his own knife, dramatic tension almost always goes up, not down, and an auto-kill with already established murderous intent means no dramatic tension whatsoever. So, neither by fiction nor by rules is an auto-kill supported.
Now, in the Going Aggro example, on a 10+, the choices are suck it up or cave. All well and good, these. If the result is 7-9, though, we see the payoff of Rouge having maneuvered Fifi into position. The additional options, at least in theory, that are available to Fifi in such a case are:
• get the hell out of Rouge's way
• barricade himself securely in
• give Rouge something he thinks she wanta
• back off calmly, hands where Rouge can see
• tell Rouge what she wants to know (or what she wants to hear)
Fifi's problem is that to do any of these, he has to, you know, do it. How is Fifi gonna do any of these, drugged and unconscious as he is? So, he's back to sucking it up (if he wants to live) or caving (if he wants to die).
Assuming he takes the damage and lives, I would, as the MC, turn to his player and say, "I'm thinking you want to defend yourself here. So, to do anything except take Rouge's next stab, you'll have to act under fire, and the fire is that drugged stupor you're in," and go from there.
As for the realism of knife to brain, assuming it went in as planned, there have been people who have made full recoveries from having chunks of their brains knocked out, so one stab through the eye isn't necessarily the end.
Charles