In many ways, this is the guy that starts the whole crime-spree rolling.
Sure, the cops are responsible in that they put five known felons in a lineup and a jail cell together. But McManus is the man who unites them and suggests that they get a little revenge on the cops. He tells them that he and Fenster heard that two corrupt cops are escorting an emerald smuggler into New York and they want to jack his jewels. From there, it's just a matter of convincing Keaton, and the whole gang is in.
Verbal describes McManus as a "good guy" but "crazy." We see a little of this side of McManus, when he's holding a gun to Kobayashi's head and arguing with Redfoot.
Later, as he prepares to start gunning down Argentinean and Hungarian gangsters, he sings a little song to himself:
"Old McDonald had a farm, ee-aye, ee-aye, oh. And on this farm he shot some guys. Ba-da-bip, ba-da-bing, bang-boom."
Maybe he's singing this to calm his nerves, or maybe he just likes shooting people and is looking forward to it?
In the end, McManus proves yet another of Söze's casualties…after shooting Hockney, and Arturo Marquez, Söze stabs McManus in the back of the neck, leaving the knife in.
If McManus had only listened to McGruff the Crime Dog...