Okay so just to clarify a few things I keep seeing people freaking out over.
First of all “Dean’s love interest” has not been referred to as such as far as I can tell by anyone but reporters/fans - the actual quote said it was an “intense fling” (I’m not trying to give shippers faith here FYI, I’m trying to say thank fucking whoever it’s not a romance.)
Secondly in the trailer. I don’t get the impression that Dean is actually trying his best to kill Sam. Assuming he has half the power Cain had we can assume he could kill Sam easily and that Ruby’s Knife poses no threat to him at all. So my guess is he is toying with Sam for fun. Similarly calling Sam a monster - toying with him.
When people don’t care, other people exclusively become a source of amusement for you to torment, and I get the impression that is very much where the demon!Dean character has gone mentally.
Side note on the fling here, if demon!Dean is as womanising and violent as they’ve told us, I’m concerned for this new female characters well being. Which I’ve seen pretty much no one talk about, but demon!Dean seems to be a really bad guy, little to no redeeming qualities from what we’ve seen. It just bodes poorly for the woman yeah?