Christian Bale's portrayal of American Psycho's Patrick Bateman wasn't inspired by a serial killer but by a Tom Cruise interview.

Source: https://www.cbr.com/american-psycho-tom-cruise-inspiration-behind-patrick-bateman/

There is even the added aspect of Bale's insecurity added to the role.

Source: https://www.cbr.com/patrick-bateman-sigma-status-explained-american-psycho/

When in fact, both filmmakers agree that the ending did happen in real life, and actor Christian Bale's Bateman won't be prosecuted for his crimes based on his affluence and high-society station.

Source: https://screenrant.com/american-psycho-ending-explained-meaning-what-happened/