Could be that he doesn't recognize her if she looks different the last time they were together. But i was also thinking thay this girl is just making the mc kill random people too. Wonder if he'll get a blowjob or a footjob
Could be a gang rape situation involving the Girl/Boy's mother. If the MC character, high-school crush, was raped (14-17) soon after losing contact with our Male character. That's enough motive for our rape baby to mature enough to go on a killing spree.
Also, as the story develops, knowing that he/she is the son/daughter of the high school love interest, be the motivation for MC to keep killing. Gender is still up in the air, I think. His/her clothes are baggy enough to hide stuff.
Also Symbolism, can't forget the desperate guy's motivation.