Find out how you can tune into the one-hour "TMZ Investigates: Luigi Mangione: The Mind of a Killer" for free if you don't have cable.

TMZ Investigates looks into Mangione's dark side and why a privileged man may have turned to murder in the special airing on Monday, Jan. 6.

TMZ "speaks to several sources who say, despite his charismatic personality, Mangione also displayed a dark side and uncovers shocking new details ..."