California v. Roy

A California court convicted respondent Kenneth Roy of the robbery and first-degree murder of Archie Mannix. The State’s theory, insofar as is relevant here, was that Roy, coming to the aid of a confederate who was trying to rob Mannix, helped the confederate kill Mannix. The trial judge gave the jury an instruction that permitted it to convict Roy of first-degree murder as long as it concluded that (among other things) Roy, “with knowledge of” the confederate’s “unlawful purpose” (robbery), had helped the confederate, i. e., had “aid[ed],” “promote[d],” “encourage[d],” or “instigate[d]” by…