I spent way too long puzzling over this one, because the reveal is a classic 'right under your nose' situation. In the 'Fear Street' book 'The Wrong Number', the Humpty Dumpty Killer is actually a pair of brothers, Jason and Duane. They're the ones terrorizing Shadyside with that creepy nursery rhyme gimmick. It's a pretty clever twist for a teen horror novel from that era, making the killers seem almost omnipresent because there are two of them working together. The whole thing plays on the fear of not knowing who to trust, since they appear as relatively normal high school students.
R.L. Stine loved these dual-identity reveals, and this one works because the nursery rhyme motif distracts you from looking at characters who seem too obvious or too bland. Looking back, it feels a bit dated, but the execution of the mystery—the way the phone calls and threats escalate—still holds up. The brothers' motivation is fairly standard jealous/resentment stuff, but the delivery system, with the rhymes and the constant looming threat, is what makes it memorable. It's less about a shocking motive and more about the sustained paranoia of having killers who could be anyone, even the guy sitting next to you in class.