I was surprised that it had such a dark tone. The violence is actually quite strong even by today's standards, though not much of it is shown. The execution of Putty Nose, in particular, is jarring. While not as hit-heavy as modern gangster epics, Tom is no slouch. (He even executes the horse responsible for a friend's death.) His final act of violence, exacting revenge for a hit that kills his life-long friend, is particularly powerful visually. Wielding two .38s, he charges into a building full of enemy mobsters to take down the boss. The audience is left guessing out in the rain, with only the sounds of gunfire to inform us of the action taking place inside. Seconds later, Tom stumbles out, severly wounded, and stumbles through the rain into the gutter. It's good heavy rain, the kind Kurosawa loved to use (Rashomon and Seven Samurai come to mind). Sam Mendes used it well in Road to Perdition also.
I'm still trying to figure out what purposes rain is used for in movies. In Public Enemy, it is used to isolate the main character, to frame him, to reveal the truth about his character. "I ain't so tough." He is truly alone for the first time in his life, and he realizes what he really is. In Rashomon, again, truth is revealed. It is a longer process, as the different stories are told, but ultimately the truth is outted and Takashi Shimura's farmer finds redemption and solace in new and untainted life, that of the child. In Seven Samurai, Kikuchiro redeems himself and proves to be made of samurai stuff after all. All in heavy, pounding, flooding rain. Rain that washes away pretense? Sin even? Seems so.