The movie Primal Fear also come to mind...
Plot
Martin Vail is a prominent defense attorney in Chicago who jumps at the chance to represent Aaron Stampler, a young, stuttering altar boy accused of murdering the Archbishop. At first interested primarily in the publicity that the case will bring, Vail comes to believe that his client is truly innocent, much to the chagrin of the prosecutor (and Vail's former lover), Janet Venable.
Vail discovers that powerful civic leaders, including the District Attorney, have lost millions in real estate investments due to a decision by the Archbishop not to develop certain church lands. The archbishop received numerous death threats as a result. He also learns that the archbishop had been sexually abusing altar boys, including Stampler.
Introducing this evidence, while it would make Stampler more sympathetic to the jury, would also give his client a motive for murder, something the prosecution otherwise has lacked.
The trial does not proceed well for the defense, as there is considerable evidence against Stampler and public opinion holds him almost certainly guilty. When Vail confronts his client and accuses him of having lied, Aaron breaks down and transforms into a new persona, a violent sociopath who calls himself "Roy." He confesses to the murder of the archbishop and throws Vail against the wall, injuring him.
When this incident is over, Aaron appears to have no recollection of it. Molly Arrington, the psychiatrist examining Aaron, is convinced he suffers from multiple personality disorder due to childhood abuse by his own father.[1] However, Vail cannot enter an insanity plea during an ongoing trial.
Vail sets up a confrontation in court. After Venable questions him harshly, Aaron turns into Roy and charges at her, threatening to snap her neck if anyone comes near him. Aaron is subdued by courthouse marshals and is rushed back to his cell. In light of Aaron's apparent insanity, the judge declares a mistrial, dismisses the jury and remands Aaron to a mental hospital.
Vail visits to tell him this news. Aaron says he recalls nothing of what happened in the courtroom, having again "lost time." However, just as Vail is leaving, Aaron asks him to "tell Ms. Venable I hope her neck is okay," which is not something that Aaron should have been able to remember if he had "lost time." Vail points this out, whereupon Stampler grins slyly and reveals that he has been pretending to be insane the whole time. But he didn't make up the identity of Roy, he made up Aaron.
Stampler now admits to having murdered the archbishop, as well as his girlfriend, Linda, whom the cleric also had molested. Stunned and disillusioned, Vail walks away, with Roy taunting him from the cell.
http://en.wikipedia.org/wiki/Primal_Fear_(film)
