There were a number of theories floating around on this. I'd stopped following wrestling quite a while before this, but Benoit, along with RVD, were my favourites so this was a shock to me.
I don't know if you remember a bloke called Christopher Nowinski who wrestled in the WWF for a short while, but along with his degree in Sociology, he studied head injuries and in particular, concussions. He requested that Benoit's brain be examined and they found that the head trauma he had sustained had severely damaged his brain and that could alone have caused mood swings and irrational thought and onset effects of dimensia, even before the complications of steroid use. They figured that it may not have been steroid use due to the staggered times of death on the bodies, which looks to be a premeditated action, not rage induced.
As I understand it, his home life wasn't crash hot either. He and his wife had a strained marriage and IIRC their child suffered from a rare form of mental retardation. He made no secret that the death of his best mate in Eddie Guerrero had rocked him to the core.
It was a shocking event to have taken place, but a reflection of the policies of the WWF in relation to talent care. Nowinski even came out later on and said that steroid use was encouraged over natural means.