An ex-girlfriend of Ohio mass shooter Connor Betts noticed a giant red flag on their very first date — when he showed her footage from the slaughter at Pittsburgh’s Tree of Life Synagogue, she said in a new interview.

Caitlyn “Adelia: Johnson, 24, told The Toledo Blade Monday that she dated Betts from March to May, breaking up just months before he gunned down nine people in Dayton’s nightlife district.

As he showed her the bodycam video on their first date, Betts bragged about his wealth of knowledge about similar violent acts, she told the paper.

Johnson initially assumed Betts was interested in the topic from a psychology standpoint, as he was studying the subject at Sinclair Community College in Dayton.

“Do you know tragedies from every city?” Johnson asked Betts in a text conversation obtained by the Blade.

He admitted he knew “a fair bit of them” but made light of his knowledge on the subject, calling it “totally normal.”

“I’m just a fountain of joy and wonderful, not-scarring knowledge,” he said in the text.

The two met in a psychology class, parked near each other and bonded over a shared dislike for their teacher and their openness about mental illness, Johnson told the outlet.

Betts told her he was bipolar and had obsessive-compulsive disorder, Johnson said,

He once took her target-shooting at Shoot Point Blank in Dayton, she recalled. Though she did not remember the exact type of weapon they used, she said that it was a rifle.

Johnson said she started feeling alarmed in May when she joined Betts on a drive to deliver a letter to an ex-girlfriend, whom he said he dated two years earlier. He’d tracked the girl using a photo she’d posted of her house on Facebook, Johnson claimed.

The eerie letter said something along the lines of “Welcome to the neighborhood. You can’t escape your past,” Johnson said. He signed it, “Your neighbor.”

“He tried to downplay it as a joke,” Johnson wrote in a Medium post published Tuesday. “But I knew it wasn’t, so I pushed further. He admitted that sometimes he got uncontrollable urges to do things… I took that opportunity to try and show him a coping method. We pulled into a parking lot, jammed the letter in a hole under a parking block, and lit the letter on fire, watching it to make sure the pieces didn’t fly off and start a wildfire.”

At that point, she wrote, she wanted to end things but waited a few days to cool off.

In the meantime, Johnson said Betts asked her about her sex life with a man she was also engaged to at the time — as she is a polyamorist — and also whether she’d ever considered suicide. He’d previously told her that he twice held a gun in his mouth but couldn’t bring himself to pull the trigger.

“In the gentlest way that I could, I told him that I didn’t think he was ready for a relationship in the mental state that he was in,” she said in the Medium post. “I didn’t have the emotional capacity to be his therapist, and that wasn’t my job. I just wanted to be his girlfriend and it was clear that he was not in a position that that was possible.”

He simply replied, “I hope you find peace and never have to stress about anything again,” she recalled.

In a Facebook chat with a friend following the breakup, also obtained by the Blade, Johnson said that “I’m scared that [Betts] might try to hurt me or stalk me.”

Johnson told the paper Betts once called her while drunk, rambling about how he wanted “to hurt a bunch of people,” she said. Still, she’d never expected he’d carry out a mass shooting.

“He was so drunk and I just figured he was mad at the world,” Johnson told the paper. “I didn’t think he was at that point yet.”

When she found out Betts was behind the Dayton rampage, she didn’t “know how to wrap my head around that,” she wrote in the Medium post.

“That man who was so sweet to me and told me he loved me was a mass murderer,” she said. “I kissed a mass murderer.”

“I have no idea what his motivation was,” she added. “I will never know. But there are a few things that I’m certain that it wasn’t. This wasn’t a hate crime. He fought for equality. This wasn’t a crime of passion. He didn’t get passionate enough. This wasn’t very premeditated. He wasn’t a thorough planner.”

Instead, she called him “the product of a failed system.”

“A system that stigmatized mental health and recovery,” she wrote. “A system that makes the mentally ill feel broken and unworthy of help. He didn’t want to seek help because of the stigma, he just wanted to better and he didn’t know how.”