Caspar wants Bernie dead because he's scamming info on the fixed fights. Bernie is under Leo's protection because he's involved with Bernie's sister Verna. Leo is jealous so he send his guy Rug to follow Verna. Rug ends up dead. Leo assumes this is Caspar getting his pound of flesh because Leo won't turn over Bernie. A gang war is about to ensue.... and Tom creates a scene where he gets thrown out by Leo. Tom is then welcomed by Caspar. Tom then goes about arranging for Caspar and his no. 1 guy Dane to be killed, and also kills Bernie himself, thus restoring order and saving Leo. In the end even though Verna was sort of to blame for the whole thing Leo sticks with Verna and Tom won't have it, it's either her or him? I lose track of Tom's motivation, I'm never really sure why he is doing what he is doing.
I think Tom has a problem. Intellectually, he's all, kill Bernie, it's the 'smart play', keeps Caspar subservient, keeps the booze flowing, keeps the status quo. And Bernie really is scum, he double crosses anyone he touches, Caspar, Mink, Tom. He'd have been dead a long time ago if Verna wasn't running interference. And Verna is right Tom does want her. But he doesn't want to want her - it's his iron will, his loyalty against his desire, His long play, is all for Leo's benefit - but we never see his motivation, I'd guess it's in his past, when big hearted Leo took a chanve on Tom
And Verna wants him - until he sets Bernie/Caspar up, then it's 'hell hath no fury' and she goes to Leo the sap. Even after the Dane tells her Tom shot, her brother she still can't kill him, so after the Bernis/Caspar confrontation, on Tom's doorstep, she does the only thing she can to hurt Tom - marries Leo.
So Tom is divided and ultimately Tom loses everything, despite him doing the long play on Leo's behaæf, he loses Leo (actually he chooses to lose Leo) and he loses the girl. And maybe he chooses to lose Leo because of the girl.
The movie was influenced by Dashiell Hammet's novel "The Glass Key". Tom Reagan is Ned Beaumont, Leo is Paul.
Great move, great book
ETA: And I don't disagree one bit with Andy I think