His activism has been extremely modest, by Western standards: Starting in 2006, he met and prayed with Shiites, and wrote several articles calling for an end to discrimination. He was promptly called in by Interior Ministry officials, who warned him to knock it off. He refused, and was jailed for almost four months.

Image ‘I’m ready to pay with my life for my beliefs.’ MIKHLIF AL-SHAMMARI Credit... Abdurahman al Shammari

When he emerged, Mr. Shammari said, his company, a subcontractor for the state oil company, Aramco, was bankrupt, its offices empty. The workers told him they had been warned by government officials to stay away. Furious, he decided to devote himself full time to human rights work, whatever the consequences.

The police had also detained his eldest son, Adel, then 28, for five days of questioning. When Adel got home, he had changed, Mr. Shammari said, and soon began telling the rest of the family that their father had become an infidel.

On Mr. Shammari’s return from prison, Adel — who had run away — began sending him death threats by text message, he said. He asked for help from the police, he said, but got none. Adel then fled to Iraq, where he fought in the insurgency. Returning home, Adel was arrested and placed in the kingdom’s rehabilitation center for jihadis, part of a high-profile effort to de-radicalize members of Al Qaeda. But when Adel got out after two years, Mr. Shammari said, “he was even more radical than before.”

At the time, Mr. Shammari had just emerged from his second jail term, this one 21 months long. The charges in that case were vague, but they followed well-publicized protests he staged after a judge tried to dissolve a Sunni-Shiite marriage on sectarian grounds.

In June 2012, Mr. Shammari organized a trip for the entire family to Mecca, hoping to reconcile with his angry son. But as they gathered in a Riyadh lobby at 6:30 a.m. to drive to the airport together, Adel emerged from a prayer room, pulled out a pistol and shot his father. Mr. Shammari jumped behind the concierge’s desk, while two Bangladeshi attendants fled. Adel shot his father three more times, but one of his sisters managed to knock the gun sideways, so that the bullets missed her father’s vital organs.

Mr. Shammari spent 45 days recovering in the hospital. When he got out, he said, “I found no reason not to keep helping people — immigrants, women subjected to abuse, other people in need.”