The bad guy is Mickey Cohen (Sean Penn), a boxer-turned mobster who wants to make L.A. his town, with no interference from the Chicago mob. Cohen was a real guy, but not so much this version, who's more like a Dick Tracy villain that happens to be a heroin dealer too. He's sleeping with Emma Stone, so of course Wooters tries to seduce her just for the hell of it, making things a whole lot worse than they need to be while he lectures O'Mara for being reckless. It all ends bloodily - not just in gunfire, but with brutal fistfights in which every blow draws Mortal Kombat levels of blood.
That writer Will Beal is allegedly writing a Lethal Weapon 5 script makes sense; that he's doing a draft of Justice League, maybe less so if you take your heroes seriously. Fleischer, though, is becoming a consistent pleasure in my book, but then I'm that one guy who actually liked 30 Minutes or Less almost as much as Zombieland. His constant themes seem to involve people teaming up to comedically take on horrifically gory threats to their lives, and that's a not a bad niche to have - provided your goal is to entertain a sick bastard like myself. If you want to laugh in the face of death, Gangster Squad's your movie. If you were looking for accurate historical drama, go read the Oscar nomination list that came out yesterday.
[And just for fun, if you've already seen Gangster Squad, look up Mickey Cohen on Wikipedia. Totally alternate universe.]