I lived and worked in Catskill, NY, during the early-80s and mid-80s, when Mike Tyson trained there under Cus D'Amato. I actually met Mike while running near Cus's home and we became acquaintances/friends (Mike was very jealous of my calves!).
Mike was a highly insecure person, even for his age, and I think that -- more than any other factor -- prevented him from fulfilling his promise as a boxer. He also partied a lot. I did cocaine with Mike a couple times in 1985-86, when he was 17-18 years old and Cus was still alive. Smoked pot with him several times as well. I'm not sure if he continued imbibing over the years, but it did not bode well for someone who everyone in Catskill called the future heavyweight champion of the world.
Forget Cus D'Amato and Kevin Rooney . . . the person responsible for Mike's success was Teddy Atlas. Teddy Atlas quit working for Cus D'Amato in 1984 largely because Cus wouldn't make Mike abide by the "house" rules -- curfews, no alcohol, etc. -- while he enforced those rules on the other resident boxers.
Mike could be a real mixed bag: one moment a highly sensitive person, the next moment a callous bully. As a boxing analyst, Teddy Atlas seems rather obnoxious, but as a trainer and pop psychologist, I believe he could get inside Mike's head and pacify him somewhat. Without that influence, Mike went off the rails.