Maybe, does he think of himself as the demon-king? Because his friends are supposed to be the heroes but he's not, so in most isekai manga that means you end up Maou so the he believes it?
But he befriended the actual Maou though? So maybe, to protect his demon friends?
But just talking it through should work...
So, for me he want them to grow stronger (what for or why, the heck I know) so he plays the antagonist. I'm wondering if he knows how much his disparition affected them and is playing with that (and that'd be pure malice here, playing with those things like that) or if he didn't realize they thought him dead and actually doesn't care (which is not even slightly better).
I'm gonna ignore everything else which could be in this chapter~~