Are MISTAKES Allowed in SELF-DEFENSE?

You use deadly defensive force because you believe your attacker is coming at you with a knife in hand, and you kill that aggressor--but it later turns out the "knife" was merely a large, black magic marker. In fact, you were MISTAKEN about the threat you perceived. Does that mistake invalidate your legal justification of self-defense? Exactly just this mistake happened to Denver police recently, and this week the local prosecutor released his written decision on whether to charge t...