I think that Benoit was suffering from Steroid abuse... it leads to violent behavior. Comparing him today to how he looked ten years ago shows that he has beefed up amazingly... and you don’t grow those type of muscles merely by weight lifting. He almost doubled his upper body musculature. He had to have been using steroids.
I have no doubt steroids played a part, but it can't be the sole cause, the whole thing was so methodical, over the course of 3 days, it seems thought out, I am wondering what else happened to him.
Other drugs? Concussions? etc.