The Professor

Nolan's Killer is the anonymous person responsible for confronting, pushing, and ultimately murdering Nolan Hotchkiss on the roof of Thorne Hall.

Biography
Late one night on the roof of Thorne Hall, Nolan assured his would-be murderer that it was the safest place to meet, and apologized for his past actions. He told the person time that was running out and it wasn't just about him. He confessed that his sister, Taylor Hotchkiss, was alive and that they needed the person's help. The person rushed to him, and Nolan was subsequently pushed off as a result and impaled by the spiked gates below.

In The Patchwork Girl, Alison reveals to Taylor that she was supposed to meet Nolan on the roof of Thorne Hall while Taylor reveals that who ever killed Nolan might be the same person who tried to kill her.

Trivia

 * Nolan died the exact same way the Perfectionists discussed at their meeting in the woods - thrown off the roof of Thorne Hall and impaled on the gates. Interestingly, the Perfectionists heard what could have been the crunch sound on someone stepping outside the woods, which caused them to leave. The trio later confirmed someone, specifically Nolan's murderer, was spying them, revisiting the same spot in the woods and finding a gum wrapper on the floor.
 * Despite Nolan's past behavior to the perpetrator, Nolan seemed to trust this person, especially with the information that Taylor was alive. This makes his killer the only other person, and only person alive, to know she is alive and in hiding.
 * Nolan's killer is very tech-savvy, as they was able to hack Alison's phone with a fake text from Nolan asking for her help. They are smart to a point where Mona says they can pin the murder on anyone of their choosing.
 * This skill is perhaps one of the reasons Nolan sought their help into the investigation on Hotchkiss Technologies.