You did not include enough context.
(Poorly written plot summary of The Untouchables.) Warning: SPOILERS
Payne testifies in court against Capone, admitting his role in channeling money to Capone over the last three years. Ness, however, notices Capone relaxed and even smiling, despite the probability of serving a long prison sentence, and also sees Nitti carrying a gun in court. He takes Nitti out of the courtroom with the bailiff and discovers that Nitti has permission from the corrupt mayor of Chicago to carry the weapon. Ness then identifies Nitti as Malone's murderer after finding Malone's address on a matchbook in Nitti's pocket.
Panicking, Nitti shoots the bailiff and runs up to the roof, the two exchanging gunfire through the building. Eventually, Ness gets Nitti in his sights, but cannot bring himself to shoot the man in cold blood. Nitti gives himself up to Ness, Ness telling him that he'll see him burn because he murdered Malone. Stating that Malone died "screaming like a stuck Irish pig" and that Ness should think about that when he, Nitti, is tried and convicted for the murder but set free anyway. Enraged at the thought that Nitti will escape punishment for his crimes, and provoked to revenge, Ness pushes Nitti off the roof. He shouts to the screaming thug, "Did he sound anything like that?" before Nitti dies on impact with a parked car.
The bolded portions are what you included in your answer. But critical information is in the stuff you left out. Especially NITTI GIVES HIMSELF TO NESS. Here, gives himself means surrenders himself. Perhaps a better phrase would be gives himself up to. I think I've said before these IMDB summaries do not contain the clearest or best writing. The implication, then, is that after that, Nitti is brought into court and tried. Where else is anyone "tried and convicted"?