The Professor

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

Biography
Nolan, having scurried from the Welcome Back party hosted by his family, called and demanded this person meet with him. 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 tells the person his sister, Taylor Hotchkiss, is alive, before being thrown off as a result and impaled on the spikes of the gate outside. His killer's motives for murdering let alone communicating with Nolan are unknown.

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. In so, it is possible this person was spying on the Perfectionists before meeting with Nolan.
 * Despite Nolan's past behavior to the perpetrator, Nolan seemed to trust this person, especially with the information that Taylor was alive.