The fact that every single character is a criminal of some sort doesnt alter the fact that the viewers are watching a heartfelt rumination on where the lines are drawn when it comes to behaving as a decent human being. All the main characters are criminals. No one is excluded since they have all the share of killings, deception and betrayal. Tom however can be considered the most interesting character since his compromised morality is always reminded unto him.
One viewer can still grasp his very humane nature in terms of how guilt and loyalty gets in the way in times of pressuring circumstances. Though Toms gets to work for Casper and left Leo due to their same affair with Verna, he is still loyal to Leo. Tom figures out how to manipulate all of the situations so that Leo survives even if it may cause his relationship with Verna. In addition when Bernie pleads with Tom to spare him when Tom is about to kill, he allows him to escape which reflects how guilt still gets in the way.
In the end however, he chooses to kill Bernie to end the gang war. Though Tom often starts the war, becomes the architect of a web of killings and double-crosses that escalate into a bloody war for control of the city, Tom is also the one who ended it. Tom maneuvers his confused ethics. In the end due to his unpredictable nature, one can see the overall message of the film in terms of ethics.