Tom's character arc throughout the play is a circular one, since Tom is looking back on a story that has already taken place. Tom cautions the audience that his memory may be faulty, but he will try his best to tell the story as it happened. At the end of the play, we may conclude that Tom is seeking absolution or, at least, forgiveness for his actions, even though leaving his family was an act of self-preservation.
Within the memory portion of the play, which takes up the bulk of the action, Tom is seeking an escape from his life, and particularly from his mother, Amanda. He finds any excuse to go out late at night, and is on edge whenever he is at home. While Tom knows his mother is right, that Laura's best chance for a normal life is through marriage, they also both seem to acknowledge that Laura will never marry.
Tom is petulant with his mother, but consistently kind to his sister. By the end of the memory, it is clear that Tom must leave his family if he is to make anything of the life he has remaining. However, through Tom's final narration, it's clear he has always wondered if he did the right thing, as he knows Laura's life was filled with suffering because he left.