Luthor. A guy from my world who did his own share of rather dastardly things.
If he is Jerry, he's not one to be silent for long on what he's done or doing to the people from his world--Peter, Amy, or Charley might hear something. The guy is a taunter. And he would be killing others as well because I'm pretty sure he has to feed. I'm not saying we need bulletproof evidence, but was hanging out with Cas and Cas suddenly not answering phones right now doesn't automatically make him evil. It could be a case of they went out and Cas went somewhere else and then something happened to him. It could be that he's Jerry. But we need something to tell on that.