Martin Godin died Sunday evening in the hospital he'd been taken to after a suicide attempt, provincial police said.
Godin was the suspect in the murders of his ex-wife, her new boyfriend and his own two daughters.
Police said Monday that Godin's condition made it impossible to speak to him and he was never formally charged with any crime.
The deaths occurred in two different towns in the Chaudiere-Appalaches region south of the provincial capital.
Police found Godin's ex-wife, Nancy Samson, 44, and Benoit Daigle, 39, outside a home in Ste-Croix. They were already dead when police arrived.
About 70 kilometres away, they found Godin and his daughters, Beatrice, 11 and Medora, 13, in a home in St-Isidore.
The older daughter was dead at the scene while the younger one died in hospital a few days later.
Godin, 54, was taken to hospital on Feb. 1 in critical condition and never regained consciousness.
Police initially said the family drama may have been triggered by a love triangle.