The movie would have been acceptable if it had engendered more than a handful of laughs. From your determination that it is "a fairly normal character-driven piece," I presume you haven't seen it?
Honestly, the character's irredeemable nature wouldn't be an issue if the film didn't continuously ask us to sympathize with him. When he loudly and publicly calls a girl in the mall (who he date-raped, among other supposedly comedic offenses) a slut for sleeping with someone else, I get the impression that we're supposed to cheer. When he nearly murders a flasher with little if any provocation, we're definitely supposed to cheer, as the crowd in the movie literally does. Better yet, his old boss throws him the keys to his go-cart, and he rides up to the police station triumphantly with his wounded victim. What do these police officers (which he previously assaulted for no reason) do when he deposits a bleeding man (this being a supposed comedy, blood can gush everywhere without effect) who he proudly announces he shot? Why, they cheer for him, of course. Understatement not being in this film's vocabulary, the final scene before the credits let's us know that he's being hailed as a hero on the local news, which has led to his re-hiring at the mall.
A good review that expends more energy than I'm willing to invest in this garbage: http://www.reverseshot.com/article/observe_and_report