How does someone go from being a kind-hearted political activist to a heartless rapist?
Although it might be upsetting to think about, it's a difficult question that must be asked.
Tommy's downfall begins when he is attacked after leaving a church, losing an arm in the process. Truman visits his friend and can immediately tell that something is different about him—he's depressed, angry, and bitter. Tommy is just a kid "whose thin defense against hatred broke down under personal assault" (2.22.50). Like a solider returning home with PTSD, Tommy has been transformed by his trauma—and not for the better.
Things are even more complicated due to Lynne's race. Because of Lynne's whiteness, Tommy "did not even see her as a human being but as some kind of large, mysterious doll" (2.17.39). Then, after the accident, she becomes (to him) a symbol of his oppression at the hands of white people. He is unable—or unwilling—to see her as a human being.
Nothing can excuse what Tommy did, but it's important to emphasize that Tommy's actions are his own as an individual. Lynne doesn't understand this at first and associates his crimes with his race. The truth is that Tommy is a guy who has experienced a lot of evil in his life and has been transformed by it. Ultimately, his story shows us the true cost of persecution for everyone involved.