Du Pont's sexuality
He was a closet gay man, right? I don't mean to mock or make light of any of it, but all signs point to him struggling with his sexuality. Surrounding himself with young athletes, his obsession with wrestling, his extremely brief marriage, and the fatal relationship with Schultz.
The other explanation is that his behavior was entirely the product of an unbalanced mind, but it seems very likely that his inability to live as an out gay man compounded his long untreated mental illness.