Believe me, Thorne, I do agree. In the circumstances you describe, I think you would be justified in trying to kill him. I just think the odds against you succeeding before he kills you (his weapon is aready drawn and threatening, remember) are so high as to be unacceptable. The odds of him killing you after the robbery phase might be high, but not as high as that.

Amazingly, in the UK, few mugging victims are ever killed, even where the mugger is armed. (Actually, I don't think it's at all amazing because the mugger is hyped up and simply wants to escape before Stealth694 happens on the scene.)