@CronosusCZ I don't think it's so much jealousy anymore as it is him slipping into his old ways. Sure, back in Cross Reverie it was jealousy because his social anxiety would get in his way, but now that he's Diablo he has Rem and Shera fighting over him basically daily. If it is jealousy at this point. It's not jealousy of them being a couple, but him jealous he can't do it so openly himself without doing the Demon Lord roleplay.
Honestly beeing angry to do something like that in a battle is truly rude and insulting is something I can truly understand ... even he is maybe angry because other reasons. And people who complain about Diablo ... do you truly expect him to change this drastically just yet. Before any of that comes, he must overcome his disorder.
@Wildfire_desu I didn't know it, thanks. However, even if it is perhaps the designer's mistake, it makes it realistic. 1) Because they are objects from a video game and therefore have absurd designs, 2) the battle equipment must be functional and not beautiful.