Question: pretty little liars the perfectionists who killed?
Answer: Mona. Mona (Janel Parrish) straight-up said she's the reason Nolan is dead, so there's that. According to her ~calculations~, she knew Nolan was dangerous enough to “push another student to the point of snapping.” By not telling his mother, Claire Hotchkiss, the truth about her son, Mona thinks she created a killer.