Ok so here's my Anti theory.
Dr. Schneeplestien created Anti by accident during an experiment on human DNA modification. Anti is not human. He’s a demon. A parasitic science experiment gone wrong. He posessesd @therealjacksepticeye at Halloween but he never left. He’s still in control. But now he’s posing as both Seàn and Jack. Every now and then Jack regains control long enough to leave cryptic messages. Anti finds out and regains control, changes them to fit a different agenda in which Jack is fighting off anti instead of the other way around and plays it cool acting like nothing is wrong. As for Chase, he’s another experiment Schneeple made that became the good guy the hero so to speak. He was trying to plan a way to expose Anti behind the scenes so Anti killed him.
That’s what I’ve got.