Geary belonged to Michael, just as Questadt belonged to Roth. Also, despite the crude insults and snubs we see toward Michael and his family at the beginning of the film...he really didn't do anything to warrant being whacked. Finally, as Cardi said he was far more valuable alive than dead.

Aside from all that...Michael probably knew that it would be quite stupid and unproductive to have the blood of a U.S. Senator on his hands.

Apple