I agree. Somewhat predictable, but well written. The story flowed and keeps the reader interested. More suspenseful could really bring the fear level up. Having the boys go into the house and recognize the body, not a bad thought. But with the boys knowing that their "friend" is dead, how would they be able to live with that without saying something to either their parents or to the police?
There are several avenues that can be taken with this story, if you were to add more to it. Meaning, the boys find their friend in the house, confront Hader, like they did outside of the house. The boys run to the police and try to explain that Hader has killed a friend of theirs, but of course the police do not believe this and feel that it is a "Halloween prank", and tell the boys to go home.
One by one, each of the boys in the group are haunted by Hader. The only way they can prove to anyone, or the police, that Hader has committed murder, is by having Hader get caught in his own lie.
Just a thought. Otherwise, the story reads well, but it does end with the reader knowing that Hader is the maniac.