The Element of Crime (1984)

Fisher, an ex-detective, decides to take one final case when a mysterious serial killer claims the lives of several young girls. Fisher, unable to find the culprit, turns to Osbourne, a writer who was once respected for his contributions to the field of criminology. Fisher begins to use Osbourne's technique, which involves empathizing with serial killers; however, as the detective becomes increasingly engrossed in this method, things take a disturbing turn.

Source: TMDb