The girl, who dated the older of the pair, claimed the teenager wanted them to emulate Mickey and Mallory Knox, the lead characters of the 1994 film Natural Born Killers, by murdering her parents and running away together.
The teenager and his co-defendant, both now 15, are on trial at Leeds Crown Court accused of planning a mass killing at their school in Northallerton, North Yorkshire.
Giving evidence via a pre-recorded police interview which was played to jurors on Friday, the girlfriend of the older boy claimed that he appeared outside her bedroom window in October 2017 with a plan to go "full NBK", a reference to the Natural Born Killers movie, directed by Oliver Stone and starring Woody Harrelson and Juliette Lewis.
Telling how she rejected his suggestion, the girl said: "He had told me on Instagram that he wanted to go 'full NBK', but I didn't really know what it was.
"Then he starts going on about a plan to kill my parents, and we would run off and it would only last a month or so.
"The stuff that he was saying to me were becoming increasingly darker, and that wasn't what I wanted in my life any more."
When asked whether she believed he would have killed her parents had she agreed, the witness said: "I honestly think he would have done."
Prosecutors allege that the two defendants had been inspired in their alleged plan to carry out a shooting at their school by their obsession with Eric Harris and Dylan Klebold, the two teenagers who killed 12 students, a teacher, and themselves at Columbine High School in Colorado in 1999.
During the interview, which was recorded on October 28 2017, the girl claimed that, prior to "recruiting" the other boy, her former boyfriend wanted her to be "his Dylan Klebold".
She said: "I hate to say it, but he was interested in Eric Harris. He wanted to be Eric Harris, and he was looking for his Dylan Klebold.
"He was this guy, who would talk to me about how we were going to shoot up a school and what I would wear and what he would wear, and how we would run away and get married and commit suicide together."
She also claimed that the older teenager had researched information on building "chlorine gas bombs" and had an in-depth knowledge of his school's security system.
The boy broke down in tears as the court heard how he was supposedly an "outcast" with a desire to "make society pay".
Jurors also heard how he allegedly had his Instagram page removed after repeatedly posting images of "dead babies" and "live streams of suicides".
"He told me that he loved scaring people, because it made him feel like he was finally worth something," his former girlfriend said.
The boys both jointly deny conspiracy to murder in relation to the alleged plan to kill pupils and teachers at their school.
They have each also pleaded not guilty to a charge of intentionally encouraging or assisting an offence and a charge of encouraging or assisting an offence believing it would be committed.
The older boy denies aggravated burglary, as well as unlawfully wounding the former girlfriend.