You build up an unreal tolerance.

Reports are that he was weaning off them. Its very common for this to happen when people do. They use a lot, then go to very little, then have a bad day and go on a binge. That binge kills them because their tolerance is nowhere near what it was just a month or so before.

Its the most common way for addicts to die.

Holy shit! Just the shock of the withdrawal could probably do you in, at those levels.