It could be about a malevolent entity. Maybe 'it's him' refers to a ghost or a monster that has been haunting a place. For example, in many horror stories, there's a particular character or presence that is the source of all the fear. This 'him' might have a tragic backstory that led to his malevolence.
There could be some indie or regional horror stories with this title that haven't gained mainstream popularity. It might be a story shared among a small community or in a specific horror - loving circle.
One of the scariest parts in 'It's Him' is the build - up. As the story progresses and more and more strange things happen that are associated with 'him', the tension mounts. For instance, if objects start moving on their own and the only explanation the characters can think of is 'it's him'. And when the final confrontation with 'him' is approaching but you still don't really know what 'him' is, that's extremely scary.
Yes, 'It's Him' can be a very scary story. The sense of an unknown 'him' creates a lot of mystery. Often in such stories, the vagueness about this character makes our imagination run wild. We start to think of all the possible threats this 'him' could pose, like being a stalker or a malevolent spirit. This ambiguity and the potential danger associated with the unknown figure are what make it scary.
Perhaps his actions. If he is doing something really evil or abnormal in the story, that could make it scary. For instance, if he is a serial killer in the plot.
It's a matter of personal perception. If you're easily spooked, you might find Lisey's Story quite frightening. But if you're used to horror, it might not seem that scary to you.