While Adrian and Don's tragic story is short, it also paints the town of Derry, Maine exactly how it should be seen in the opening pages of It. While Adrian’s death (devoured by the monster hidden as a clown, Pennywise) shows you the supernatural horror rotting away at Derry’s core, it also shows you the true nature of Derry: cold, shrivel, and cruel. While Pennywise may have finished him off, the true evils are the town’s people, from the low lives that harass the men, to the police force that doesn’t respect Adrian’s grieving boyfriend. Pennywise is horrifying, but Derry is far worse and far more real.
Adrian’s death starts off the second wave of death the Loser Club must deal with, and it’s unclear if these characters will be in the movie.