Primal Fear (1996)

When a young man, Aaron, is charged with the horrific murder of Archbishop Rushman, hot-shot Chicago lawyer Martin Vail takes on his defense at no charge. Aaron was a homeless street kid before he was taken in by the Archbishop. He's shy and speaks with a stammer. Vail is convinced that Aaron is innocent but after discovering a video that shows Aaron may have had good reason to want the Archbishop dead, he begins to question that conclusion. When Aaron lashes out at the psychologist examining him another personality, Roy, is revealed. With the trial already underway, Vail cannot change Aaron plea and so has to find a way to introduce his client's condition. Aaron has something of a surprise for him as well.