Disillusioned to the world around him following the rejection from his crush, an introverted youth finds solace in an online community; however, as the site becomes an echo chamber for hateful rhetoric, his spiritual ennui morphs into a violent ideology.

This sounds more like a good setup (or even first half) to a dramatic or dark coming of age tale, not a horror story. If so, consider starting the next attempt by glossing over most of this one. For example, “After being influenced with a hateful ideology, a disillusioned fifteen year-old…”

If it’s character-driven, then it may be more about what he deals with than an objective he actively seeks to achieve. Like, how his family and friends react to the change in him.