The Hidden (1987)
It's only human on the outside...
A diligent cop finally tracks down and arrests a businessman who has suddenly, inexplicably turned into a serial killer. Figuring that the case is closed, the policeman is surprised when a mysterious FBI agent becomes involved with the investigation. It soon appears that the perpetrator isn't quite what he seems to be... and even though he's near death, he's found a way to make sure that the killing doesn't stop.