In Foxcatcher, we see John du Pont and Mark Schultz build some kind of friendship, based primarily on Mark's need of stepping out of his brother's shadow and John trying to impress his mother by achieving success his own way
They also seem to share, each in their own manner and for different causes, a feeling of loneliness which appears to bring them even closer
Afterwards, when John finally persuades Mark's brother Dave to join his wrestling team, that friendship is deeply affected, to the point of making Mark isolate himself and stop all contact with John
In the end, we see John watching the documentary where Mark appears regarding him as the one responsible for his success. Suddenly, John asks for his car and goes straight to Dave's where he shoots him right after asking him:
You got a problem with me?
Now, I know that this film is based on a true story, and therefore we can only speculate over what really motivated du Pont. But considering the adaptation as we have it, could we say that he felt bad about the end of his friendship with Mark and considered Dave the responsible?