What broke up the Beatles?
Well, if you want to put it down to one thing - that would be heroin. John's heroin addiction broke up the Beatles. Each one of them has implied that in various ways. Of course John got heroin from Yoko - so you could blame her. Of course if Brian hadn't died, John would never have gotten addicted (I think Brian would have taken care of it somehow) so you could put it down to Brian's death.
Oh and what killed Brian? Drugs.
Oh the irony.