I suspect PM was the target, with MM collateral damage. Like maybe (speculation only) AM planned to use MM as his alibi while PM was killed. But something went wrong, MM realized AM was responsible/had set it up, and she had to be silenced. Maybe PM's time of death was earlier, and the estimate of 9:00-9:30 was for MM? (i.e. PM was at the kennels, MM arrived, argued with AM, left, returned to find PM killed, was shot as she tried to run.)
I think AM had more reason to eliminate PM at that point - thinking it would put an end to the audits and review of assets in connection with the MB lawsuit. Terrible, evil, heartless logic, to be sure, but with a clear end goal - no defendant, no more lawsuit. Killing just MM wouldn't have stopped any of that. IMO.