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.

As Richiev said.? The logline sets up a situation for a plot but doesn't follow through with a plot.? As a result of coming under the influence of the violent ideology, what does he do, what becomes his objective goal?

As Richiev said.? The logline sets up a situation for a plot but doesn't follow through with a plot.? As a result of coming under the influence of the violent ideology, what does he do, what becomes his objective goal?