Why is Millhone determined to find the person who shot Magruder in the novel, O Is for Outlaw?

Partially responsible for Magruder's downfall, Millhone discovers she's made a mistake and hopes to make up for it by finding the man who shot him.

".... if I'd been an unwitting accomplice to his [Magruder's] downfall, I needed to own up to it and get square with him."