Tom Reagan is a man so conflicted over his loyalty to his boss that he becomes convinced that he has no heart. That is the central problem of Miller's Crossing, and the movie, to its credit, does not try to provide us with a simple answer to it.
It has been said of the Coen brothers that their movies are overly stylized. Relative to the styleless bulk of movies made these days, that's probably true, but in absolute terms it's a false reaction to their work. Goes to show how low our standards have fallen. We're so used to seeing pictures with no cinematic style at all that when we see something like Miller's Crossing we're distracted by the shiny surface.
The tone of this picture is far more serious and reserved than anything else the Coens have done. It's a brooding piece of work, and it shows the breadth of their talent. Well done, very well done.