I can't remember what it was called but in a doco that was released within the last year about the murders, one of OJ's friends (possibly management/agent) said that after the trial, some time after, OJ actually said something to the effect of 'if nicole hadn't answered the door with a knife in her hand this never would happened, or she'd still be alive' or something like that. Hard to believe he'd confide that if he was innocent. Of course the person could have made that shit up too.
The reality is he had a LONG history of abusing Nicole. That's a fairly common precursor to the ex (or even current wife) being murdered. The idea that she was banging the son in law is an interesting one. The only thing that makes me think it didn't happen is the history of violence with OJ, she wasn't THAT suicidal.