Why does John du Pont do that to Dave Schultz?

I'm afraid I can't give you a straight forward answer but I'll share the info I've gathered on John E du Pont, I've asked myself the same question after watching the movie.

If you're looking for a factual reason why Du Pont Killed Dave, here's what wiki says about him,

In the 1980s, he established a wrestling facility at his Foxcatcher Farm after becoming interested in the sport and the pentathlon events. He became a prominent supporter for amateur sports in the United States and a sponsor for USA Wrestling. In the 1990s, friends and acquaintances were concerned about his erratic and paranoid behavior, but his wealth shielded him. On February 25, 1997 he was convicted of murder in the third degree for the January 26, 1996 shooting death of Dave Schultz, an Olympic champion freestyle wrestler living and working on du Pont's estate. He was ruled to have been mentally ill but not insane and was sentenced to prison for 13 to 30 years where he died at age 72 on December 9, 2010.

By this, I convinced myself that his mental illness led him to kill Dave Schultz.

Also, in the movie, there are few details you might want to consider regarding du Pont's unpredictable behavior.

1. Spending money superfluously, like he writes a check of $ 10,000 when Marks wins in Olympic trials.

2. Inferring to Mark that he is no good at wrestling without his brother's help or training. Made Mark believe that his brother gets all the credit for Mark's hard work.

3. Drug usage.

4. Being friendly one day and in the next, calling Mark an ungrateful ape and disrespecting Mark in front of his team by slapping him.

5. Not having a healthy relationship with his mother. Desperately trying to prove that he's a good wrestling coach to his mother and in one scene his mother leaves in the middle of training.

6. Dissing army officers that he did not get the machine gun (or a weapon of some sort).

Purely my personal interpretation

From the movie I interpreted that John E du Pont wanted Mark Schultz to win in the Seoul Olympics and wanted to be on his side as coach during wrestling matches but it did not turn out that way. Maybe he was unable to take one loss after another like losing his mom and losing in Olympics and he showed all that pent-up anger on Mark's brother and wrestling coach.

Even though he did not have a stable relationship with Mark, he again enjoyed his praises for du Pont at the ending scene and was wondering why he did not receive similar from Dave who has more respect in his eye. Since he also did not get the similar attention from his mother and never achieved that, he got furious when the same neglect was given to him by Dave.

Also, du Pont may have even seen the reaction of Dave when he says du Pont was a mentor to him while watching the documentary. So he might have compared Mark's reaction and lines in the documentary with Dave's. He might have also thought that he gave everything to USA wrestling and still it was not enough to get respect from Dave.

He could not hold of himself due to his mental illness and killed the poor guy. That is my interpretation of the events.

I thought it was because at one point the interview guy told Dave to say something nice about John, something that would make it seem like John was the reason for the team's success. This was obviously something John wanted Dave to say seeing as it was his documentary being made, but Dave doesn't. He just keeps shaking his head and says "I don't know what you want me to say." Then after John watches the documentary, he sees that Dave doesn't give him any credit like he wanted. So he goes over to shoot Dave believing he must "have a problem" with him.

I've done a little research on the true story and the movie doesn't really tell the truth. Mark only lived in Foxcatcher for 2 years, while Dave lived for almost 7 years. Du Pont had a much closer relationship to Dave than Mark. Dave only actually went to Foxcatcher Farm after Mark had already left. They never lived there at the same time. In 1996, Dave said that he would leave the farm after the 96 Olympics and that made Du Pont start acting stranger and stranger and that's the main reason for the murder.

Just because Dave might've failed being a coach or was the problem of the fallout between Mark and John, it doesnt mean you should take someone's life like that. So obviously the answer here was that he had mental illness to begin with, and since his mother died and losing at Seoul that just simply enhances everything. Moreover Dave didn't give good acknowledgements in John's own doc. Hence John went full retard. "Never go full retard". Case closed.