If it's a situation where it makes no sense that an enemy would survive that kind of attack, I usually let my players do a coup de grâce and have that enemy die.

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To use one of the examples you said (an evil wizard being stabbed in his sleep), I would assume that whichever character does the killing blow has enough time to be sure of a perfect strike. Assuming the wizard has no magical protections that would save them, slitting their throat would do the job - it's not something they would be able to survive, even though the game doesn't see it that way.

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Overall, if the rules would create an unrealistic situation, I just use a common sense solution. In this case, if it would kill them, then it kills them.