A former high school classmate of Dayton mass shooter Connor Betts says she called cops on him when he bragged that he put her on a sick “rape list” he created — and blames a failure to hold him accountable back then for his weekend rampage.

“I was not surprised at all when I heard his name on the news,” Jessica Masseth told the Daily Beast of the 24-year-old gunman who killed nine in Dayton, Ohio, early Sunday.

“We predicted he would do this 10 years ago.

“If he was only held accountable, this shooting would have never happened because he wouldn’t have been able to buy any firearms.”

Masseth said students at Bellbrook High School in Ohio knew in 2010 that Betts had a “rape or kill” list in a notebook — and says he even texted to her while describing in detail “what he wanted to do” to her.

“The lists basically had any girl who turned him down, any girl who thought they were above him, and any guy that was competition or was seen as a threat,” she told the site, saying it showed a “God complex mixed with ‘Iwantattention.’”

Her mother persuaded her to go to the police, and Masseth says he was questioned and suspended from school — but allowed back the next year, which Masseth dismissed as a “slap on the wrist.”

“In the texts, and on the lists, he talked about destruction and dismemberment. I mean, how did the police not know he was going to do something like what he did this weekend?” she told the Daily Beast.

“Everyone knew he was not right.

“Everyone in high school knew what he was capable of, but there was a gap in getting Connor help because people just decided they didn’t care.

“If people just cared a little more, none of this would have happened.”

She angrily posted on Facebook about the “levels of failure here that are sick,” according to the Daily Mail.

“The school failed us. The police failed us. He spoke and wrote of rape, decapitation and just the total destruction of those on that list, which included me,” she reportedly wrote.

“This could have been prevented 10 f—ing years ago when I made that phone call, when I was interviewed by the police and when I told the school.”

Betts killed nine people — including his own sister — and wounded 27 more in his 30-second rampage just after 1 a.m. Sunday in Dayton’s popular nightlife district, before he was shot and killed by cops.