Algodoo

Killer: Celeste

Reason: Celeste is the only person in the group with powers (If you don't count flying as a power, which I don't) and even though he says he doesn't want to kill anybody, he seems like he's only one who would read about powers and actually have powers. Celeste must have knocked out Teal with the pipes and to make sure that Teal wouldn't wake up and that nobody would find him, Celeste stabbed Teal in the back as soon as he woke up and dragged his body down to the secret passage way so that nobody would find Teal's body. But he forgot to put the book back the way it was, so blue? was able to find the passage and Teal's body.