No, not absolutely positive and if I used those words (I don't think I did) then I withdraw them. Reasonably certain is, I believe, the criterion most legal systems apply. Consider post 121 above. Suppose man walks towards me in a manner I consider threatening, openly wearing a gun in a holster. How is that different from the man with the knife you describe? Am I entitled to kill DuncanO'Neill in case he wants to shoot me, or should I wait until I am sure he intends to? He might simply be exercising his right to bear arms, and have an unfortunate look in his eye.
I am not trying to take away the right of self defence, I am trying to place limits around it so that the intended victim can protect him/herself without committing a worse act than the attacker. I perceive that to be a very real danger, and I read a desire for just that in your posts and those of others.
I agree that a dead rapist is never going to be a repeat offender:that's a trite truism. But a dead passer-by is never going to be able to do anything ever again, is s/he? Ever! That's why anyone who contemplates using "ultimate force" under any circumstances must accept the consequences of his/her actions, and if that force is misapplied, that person must pay a very heavy penalty indeed ... the same penalty as any other murderer would face.
(I believe murder would be the appropriate charge, rather than a lesser one of (say) manslaughter, because anyone who carries a weapon knows it is a lethal instrument, designed to kill and with no other purpose. They must realise that if it is used, death is likely to result: killing is clearly within that person's contemplation before the event.)