Something I think would be good for some extra spice, is to make them think that Rimuru and his comrades are akumatized. Since they aren't human, and the kids aren't familiar with the non human races in Tensura, having them think the monsters are akumatized people would put an interesting twist
He wanted to save them mainly and if it didn't work he'd still have his revenge it's a win win situation he could not have lost anything by killing them