What was the cause of Holy Mother's death in the novel, The Washington Square Ensemble?

Indicted for fifteen of the thirty-five or more murders he committed, he was convicted of three and imprisoned at Attica. There Holy Mother not only witnessed but inadvertently became involved in the Attica riot, during which he was severely injured. In the hospital he was given morphine to ease his great physical pain, but the narcotic also unleashed nightmares in which he re-lived the most brutal and gruesome murders he had committed; only larger doses of painkillers could block out both kinds of pain, but they led to heroin addiction and eventually — when he could no longer endure the situation — to his death.