He says his father -- Sheikh Nimr al-Nimr -- had become too much of a prickly thorn in the side of Saudi Arabia's rulers.
The firebrand Shiite cleric was steely in his pursuit of justice and dignity, says his son, and had exposed human rights abuses in the kingdom. For that Nimr al-Nimr knew he would pay the ultimate price.
"He spoke loudly about their tyranny. So they killed him," says Mohammed al-Nimr, 28, who calls himself a believer but says he is not religious like his father.
"We are so proud of him, and we are not going to be intimidated or stop asking for freedom. My father stood peacefully for the lives of all people."
The new year had barely begun when Saudi Arabia announced the executions of the cleric
and 46 other prisoners, stirring global uproar over what Amnesty International called an "utter disregard" for human rights.
A violent protest at the Saudi Embassy in Tehran over Nimr's beheading prompted Saudi Arabia to sever ties
with its rival across the Persian Gulf, deepening sectarian rifts in the region
In a statement, the Saudi government said the prisoners had received fair trials. They were terrorists, the Saudis said, and the executions were carried out in the interest of national security.
But critics of the regime, including Nimr's son, saw the cleric's state-sanctioned killing as a way to crush dissent in a nation that abides by the strict interpretation of Sunni Islam known as Wahhabism. Differing ideologies are not tolerated, and Saudi Arabia's minority Shiites are seen as heretical.
The Shiites -- 2 million out of 18 million Saudis -- live mostly in Saudi Arabia's oil-rich Eastern Province and say they have been marginalized in every way. Mohammed al-Nimr recalls a time when he and his family were prevented from praying while on their way to the western holy city of Mecca.
"If we traveled outside Eastern Province, we were scared," he says. "We were treated like criminals. We were infidels to them."
In fact, he sees little difference between the Saudi regime and ISIS.
"It's not about Shia or Sunni. It's about tyranny," he says. "They are using religion to gain influence in the region.
"ISIS is the same exact picture. It's the same tree," he says. "In Saudi Arabia, the tree is trimmed because they have money. So they have a better image. But if you compare them, they do the same thing. They have the same ideology."
A request to the Saudi press office in Washington for comment was unanswered at the time of publication.
The younger Nimr moved to the United States from Saudi Arabia five years ago, pursued his education and recently earned a degree in mechanical engineering.
In Saudi Arabia's Eastern Province, his father continued his quest to gain equal rights for Shiites and other religious minorities in the kingdom. Resistance, his son says, ran in the family -- Nimr al-Nimr's uncle who led a revolt against government tax collectors and orthodox Sunnis.
As a young man, Nimr al-Nimr spent a decade studying in Iran and later in Syria, his son says. Mohammed al-Nimr was born in Syria and says his family moved back to Saudi Arabia, to their ancestral town of Awamiya, when he was in the second grade.
He remembers walks with his father among the palm trees at sunrise, before morning prayers.
"He always had good advice for me," Mohammed al-Nimr says. "On how to improve myself. How to be good to others. How to make positive change."
Nimr al-Nimr began attracting attention in 2003 when he was detained for leading prayers in public and then in 2007 when he sent a petition to the government demanding economic and political freedoms.
He called for the secession of the Eastern Province and for Shiites to seek assistance from Iran if they found themselves embroiled in conflict, according to U.S. diplomatic cables published by WikiLeaks
"Our dignity is more precious than the unity of this land," he said in a 2009 sermon.
In one of the cables, a U.S. State Department official who met with Nimr described the cleric as a maverick with little influence. Nimr told the official he was portrayed as being "more radical than the true content of his words and beliefs" and that he "espoused conciliatory ideas." They included American ideals such as liberty and justice.
Nimr gained popularity with the advent of the Arab Spring. His thin, wiry face, framed by salt-and-pepper hair and beard, became better known as thousands followed his call to protest.
He was detained in 2012 after he was seen celebrating the death of a Saudi royal in a video circulated on social media. His nephew, Ali al-Nimr, then 17, had already been detained for taking part in demonstrations and is currently in solitary confinement awaiting death by crucifixion.
At the time, Mohammed al-Nimr was with his mother, dying of cancer, in a hospital in America. They learned the Saudi police shot the cleric several times in the leg before his arrest. Saudi authorities said Nimr was injured in a shootout.
After that, Mohammed al-Nimr returned to Saudi Arabia several times to visit his father. The last time was in the summer of 2014, when he went with his sister Zainab to a prison in Jeddah.
Their father was brought from his cell to a visiting room. He was wearing a white dishdasha (a traditional Arab robe), and his left leg was shorter than his right. His children say that's because his gunshot injuries were not treated appropriately after he was shot.
Nimr al-Nimr smiled and told his children he was thankful to God. He encouraged them to make fruitful plans for their lives, to craft a better future.
Mohammed al-Nimr remembers his father mentioning human rights activist Waleed Abulkhair
, sentenced to 15 years in jail on what Saudi authorities said were charges of "insulting the judiciary, disobeying the ruler, and harming the reputation of the kingdom."
"This guy is from a wealthy family and he is for want of nothing. Even so, he stands up for everyone. He is a role model," Mohammed al-Nimr recalls his father saying.
At that last meeting, Nimr al-Nimr told his children that nothing would happen unless God wanted it to happen. But even though he remained in high spirits, everyone in the room sensed what was to come.
"He knew they would kill him," his son says. "He was steadfast that he would not change his statements; he would not acquiesce to them even at the threat of death. He was not afraid."
It has been almost three weeks since his father's execution. The family is still waiting to bury him.
"We don't know what happened to my father because they never returned the body," his son says. "They should return him so his aunt, his brother, his mother can have a proper burial for him and have the chance to say goodbye."
The son rarely spoke publicly before but now says he has broken his silence to spread his father's message. It is more crucial than ever, he says, in a Middle East torn apart by violence and deep sectarian rifts.
Mohammed al-Nimr wants Muslims to hear his father's words: You cannot win the world with violence.
"My father used to tell people that the roar of the word is mightier than the sound of bullets," he says.
Mohammed al-Nimr says he misses his father's smile more than anything. Now that the initial shock of the execution has passed, he takes solace in knowing his father is in a better place. Nimr al-Nimr, says his son, has become a martyr of this era.