Dee Snider just unleashed a bitter rant about Twisted Sister drummer A.J. Pero's death -- saying his heart attack was totally avoidable, but Pero did nothing to save himself or protect his family.
Dee came on TMZ Live to talkTMZ Live to talk about the band's upcoming farewell tour, but ended up venting about Pero. Dee told us he's in the anger stage of grieving because ... he says Pero knew he had a history of heart disease in his family, but never got checked out.
A.J.'s autopsy results -- which Dee revealed -- are what really set him off about his friend's death.
Watch him unload ... he's a man on a mission.