What is the test for a defendant to be found guilty of assisting and abetting murder?

Permitting aider and abettor liability to be predicated on intent formed after the murder was completed constitutes an invalid legal theory. Such an error generally requires reversal, "absent a basis in the record to find that the verdict was actually based on a valid ground." (People v. Guiton (1993) 4 Cal.4th 1116, 1129, fn. omitted.) Had the issue before the jury been whether defendant directly aided and abetted murder, the rule of reversal might well apply here.²³