He was a churchgoing ex-football player and honor student who wanted to join the Marines — but instead became a suspected mass killer.

Unlike other recent mass killers, there were no big red flags, no brushes with the law, no tipoffs to classmates that Texas shooting suspect Dimitrios Pagourtzis was a ticking time bomb.

In 2012, Pagourtzis was named to his middle school honor roll. In 2016 he was a defensive lineman on the Santa Fe High School junior varsity football team.

He did Greek folk dancing at his church, competed in a national history contest — and once traveled with his family to New York City, happily posing in front of the Rockefeller Center Christmas tree.

“Unlike Parkland, unlike Sutherland Springs, there were not . . . warning signs,” Texas Gov. Greg Abbott said Friday, referring to two previous mass shootings, after Pagourtzis was arrested

for opening fire in the school, killing 10.

“His slate is pretty clean.”

But secretly, the 17-year-old student was sketching out his massacre in private journals — including an ultimately unfulfilled plan to kill himself at the end of the rampage, Abbott said.

Pagourtzis was known at school as “weird” and a “loner,” other students said — one who had grown only weirder this year, when he began wearing a trench coat every day and posting odd things on social media.

“He never seemed that right,” Alex Neal, a Santa Fe freshman who used to sit next to Pagourtzis in a business class, told The Post.

“This year, what weirded me out was he started wearing a trench coat, and he started being about communism and stuff, wearing like little pins and stuff.

“I know earlier in the year he told me he was buying knives off of Amazon.”

Pagourtzis posted a photo of the trench coat and pins on Facebook on April 30, according to screenshots of the now-deleted page.

He explained in the photo what each pin represented: a hammer and sickle for “rebellion”; a German Iron Cross for “bravery”; a satanic Baphomet goat idol for “evil” and a Japanese rising sun for “kamikaze tactics.”

That same day, he also posted a T-shirt with “Born to Kill” emblazoned across it.

It was the same outfit he would wear three weeks later when he stormed the school with a shotgun and a revolver, witnesses said.

But the postings weren’t his only chilling social-media ­offerings.

On April 24, Pagourtzis posted a photo of a handgun and a knife on Instagram.

“Hi f—kers,” he wrote.

And hours before the shooting, he wrote “Dangerous Days” alongside an occult symbol, a law enforcement source told CBS News.

Pagourtzis had discussed buying guns and liked war-based video games — but “he never talked about killing people or anything like that,” said his friend, Tristen Patterson, 16.

On Facebook, the alleged killer wrote that he wanted to join the US Marine Corps in 2019 — although the military branch told news outlets Friday that there was no evidence he ever applied.

Pagourtzis rarely talked about himself at school and could be quite aloof in class, students said.

“He would get mad and move away from whatever the problem was — like last year there was this one kid that was really annoying that was sitting by us, and Dimitri would [isolate] himself,” said Neal, 15.

Some students said Pagourtzis had been bullied at school.

“He’s been picked on by coaches before, for smelling bad and stuff like that,” Dustin Severin, 17, told NBC affiliate KPRC.

“And he doesn’t really talk to very many people either. He keeps to himself.”

But other students said they’d never seen anyone picking on him.

“I played football with him for three years,” Rey Montemayor III told The Daily Beast.

“People on the news said he was bullied a lot. I never seen him being bullied. I never bullied him. He was cool to me. I lifted [weights] with him a couple of times.”

Another classmate called him “a very nice, caring kid.”

“He’s very chill,” said Logan Roberts, 18, who shared two classes with Pagourtzis.

At least one teacher said the suspected killer didn’t stand out in class.

“He was quiet, but he wasn’t quiet in a creepy way,” Valerie Martin, who taught Pagourtzis in a language-arts class last year, told The New York Times, adding that he had competed for the school in a national history contest.

Pagourtzis’ dad is the legal owner of the two weapons the teenager brought to school Friday, according to authorities.

An immigrant from Greece, the dad “liked” the Facebook page of National Rifle Association spokeswoman Dana Loesch — but he mostly posts online about his family and boats.

Among the images is one of the dad, his son and a little girl in front of the Rockefeller Center Christmas tree, posted in 2013.

On Friday, law enforcement raided a trailer near Pagourtzis’ home, and neighbors told the Houston Chronicle that they had been told by the officers that it was a “stash house” for homemade pipe bombs and Molotov cocktails.

The Pagourtzis family belongs to a local Greek Orthodox Church parish, according to a priest in a nearby parish.

“He is a quiet boy,” said Father Stelios Sitaras of Assumption of the Virgin Mary Greek Orthodox Church in Galveston, who met Pagourtzis when his dance troupe performed at a festival last October.

“You would never think he would do anything like this.”

Neal agreed. “When I first started hearing rumors [that Pagourtzis was the killer], I was sick to my stomach, shocked that he would do that,” Neal told The Post.

“When I saw it was confirmed, I was angry, like I can’t believe he would do that.”

A woman who answered the telephone at a Pagourtzis family number declined to speak with the Associated Press on Friday.

With Wires