CAIRO — The doctor’s sorrow was twofold when he found his son in the back of an ambulance, waiting to be carried in to the morgue here with a bullet hole in his chest.

Not only was his youngest son among the scores of supporters of deposed President Mohamed Morsi who had been killed by security officers on Monday, but the doctor had spent the last months of his son’s life shouting at him about his politics.

“All the time there were fights between me, him, his mother, his brother — all about the Muslim Brotherhood,” said the doctor, Samer Assem, 59.

The military’s early-morning assault that left at least 54 people dead might have been expected to unite Egyptians in grief and anger. Instead, Egypt’s bloodiest day in more than two years of unrest appeared to intensify the scarring arguments about who should be ruling the country and who is responsible for its plunge into turmoil.