Speaking solely from the web novel, as I have not read the LN, he never rebelled. There haven't been any completely crazy plot changes yet so I expect it'll remain this way.
From what I remember he's about as loyal as Shion where they are both feverishly loyal. So worry not diablo is just a mostly peaceful walking nuclear bomb.
What a horrible description for Shion's absurdly overpowered new skill.
It does a LOT more than just her food taste as good as she imagines it. She actually alters reality based on her own wishes. She decides that an enemy's 'true' state has no limbs, and they then HAVE NO LIMBS anymore. Even with healing, they just stay in their now 'perfect' limb-less state. This works with any of her attacks from now on. She can make wounds that she wishes wouldn't heal....so they don't.
And while Diablo is a pretty scary guy, he's nothing compared to
the praying mantis Rimuru names. That thing is powerful on a scale that's absurd. Like 'murder demon lords like they were minions' absurd.
To think Shion’s ability ends up being one of the most OP skills in existence lmaoo
@Riger pretty sure it wasn’t explained what the ability was truly capable of until later on. It was solely explained with its utility to change food flavor when introduced so this explanation isn’t at fault.