@Markoff said:

So DuPont was gay and could not stand there is something between him and Mark, so in weak moment he decided to kill his brother? At least that was my impression movie tries to imply, though real Mark Schultz denied portrayal of homosexual undertones.

I does seem like du Pont was gay, but as for his motive portrayed in the film, I got the impression it was because he thought Dave was undermining him (hence the "do you have a problem with me" line before shooting him). He made demands for his brother but also there's a scene where Dave is asked to say du Pont is his mentor for a documentary. Du Pont kills Dave immediately after watching this documentary, which appears to not have Dave saying this.

I was not familiar with this story, found it quite boring until for me surprising murder. Was glad to read on Wikipedia murderer died in jail at least, for taking away father from his children.

I knew the story but found the film strangely unengaging. The performances and score were good, but there was something not quite holding it together.

Do people in US not boycott products of this sick family?

To be fair I think it was just this guy who was sick in the family (well, from what we publicly know anyway).