The story revolves around a seemingly average anonymous blue collar worker as he led by a stranger in to a world of underground fighting and slowly becomes ultra violent. “Jack” the anon meets Tyler in a bar and gets in a fight with him in the parking lot just to do something out of the ordinary and becomes free. Eventually turning that bar space in to a place for other men to do the same. Through a course of hard knocks with Tyler “Jack” starts feeling pressured to more extreme actions and eventually kills his friend. Wherein we learn that Tyler never existed and was a imaginary figment the whole time.
That condition is called Dissociative Identity Disorder (DID) which is commonly triggered by childhood abuses or extreme traumatic stress that causes the mind to split in to two or more identities that have no interaction with each other. Seeing Tyler may indicate that “Jack” also suffered Post Traumatic Stress Disorder (PTSD) which in this case bizarrely mirrored the Milgram experiment.
The Milgram experiment sought to find how much pressure by an authority figure would make a person inflict harm upon another against their better judgment. Psychological motivations such as this are extrinsic; coming from influences outside of the individual.
“Jack” lets Tyler push him in to extremely uncomfortable actions that he knows is wrong but proceeds with anyhow. His own mind made up an ultimate authoritative figure to escape responsibility for his own misdoings. The end of the movie is far from a conclusion as people suffering DID will likely have another personality surface to take place of whichever has disappeared and very likely the depression of “killing” a person would make “Jack's” PTSD more severe instead the character feeling overwhelming clarity and freedom.
Another film with excellent application of psychological themes is Donnie Darko. The movie is a story about one of the most emotionally vulnerable times in life going wrong. Donnie a socially out-casted teen whose only friend is a giant deformed rabbit named Frank until he meets a girl at school. It is made very clear that is not an imaginary friend but a hallucination that he truly believes in. Hallucinations are a common signifier that a person is suffering schizophrenia but it is unknown if the character was ever diagnosed with the condition.
Merriam-Webster define schizophrenia as a very serious mental illness in which someone cannot think or behave normally and often experiences delusions. Frank was a terrible being that did devastating actions regularly and Donnie would participate with no control of the situation and often not remember what happened. Unlike a person suffering of DID he did not believe he was Frank rather accepting the rabbit was a friend and he could not