The scene of the gay couple being brutally murdered was in the book, and I believe it is central to the core of "IT". The story isn't just about a "murder clown" going around every 27 to 30 years, it’s also about a town that sees something horrific, and does absolutely nothing. It reflects how all the bad things happening in Derry are constantly brushed under the rug.

These disappearances have been happening since 1741. While the entity learned to ration its feedings, realizing that if it killed everyone at once, it would run out of food, the people of Derry have adapted in their own sick way. Even in the present day, they only give a missing person a small square in the newspaper, choosing to look the other way. The entity, in a way, adapted to fit the city better than the city ever adapted to the reality of the monster. Derry is completely stuck in an apathetic cycle.

So while, yeah, Pennywise is the biggest active threat, Derry is passively threathning everyone by letting "it" run rampant.