What is the test for vicarious liability of a aider and abettor in a murder case?

"Because aiders and abettors may be criminally liable for acts not their own, cases have described their liability as 'vicarious.' [Citation.] This description is accurate as far as it goes. But, as we explain, the aider and abettor's guilt for the intended crime is not entirely vicarious. Rather, that guilt is based on a combination of the direct perpetrator's acts and the aider and abettor's own acts and own mental state." (People v. McCoy, supra, 25 Cal.4th at p. 1117.) "Aider and abettor liability is . . . vicarious only in the sense that the aider and abettor is liable for another's actions as well as that person's own actions. When a person 'chooses to become a part of the criminal activity of another, she says in essence, "your acts are my acts . . . . " ' [Citation.] But that person's own acts are also her acts for which she is also liable. Moreover, that person's mental state is her own; she is liable for her mens rea, not the other person's." (Id. at p. 1118.)

In People v. Nero, supra, 181 Cal.App.4th at p. 507, we noted McCoy "held that an aider and abettor may be found guilty of greater homicide-related offenses than those the actual perpetrator committed. Extending that holding, we conclude that an aider and abettor may be found guilty of lesser homicide-related offenses than those the actual perpetrator committed." Similarly, People v. Samaniego (2009) 172 Cal.App.4th 1148, held: "Though McCoy concluded that an aider and abettor could be guilty of a greater offense than the direct perpetrator, its reasoning leads inexorably to the further conclusion that an aider and abettor's guilt may also be less than the perpetrator's, if the aider and abettor has a less culpable mental state. [Citation.] Consequently, CALCRIM No. 400's direction that '[a] person is equally guilty of the crime [of which the perpetrator is guilty] whether he or she committed it personally or aided and abetted the perpetrator who committed it' [citation], while generally correct in all but the most exceptional circumstances, is misleading here and should have been modified." (Id. at pp. 1164-1165.)

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c. Discussion.