MC's reaction can be summed up like this:
"My power...sometimes it scares me."
Anyways, I am rather suspicious with Bel saying he's the MC's other half- especially with him saying that he won't erase or violate the things or persons that the MC doesn't want to be erased or violated.
Those things can be interpreted in all sorts of things, that can be both awesome and terrible at the same time.
Let's say that the MC wanted to delete the entire castle, but he did not account for the non-hostiles or the non-combatants that happened to be in that castle.
Are they going to be deleted? Or worst, got sent to the abyssal dimension where the Abyss "Goddess" and demons reside?
If it was the former, they simply got erased from existence, easy.
If it was the latter....boy oh boy, those unfortunate souls are going to experience what it feels like to be demons' chew toys (or worse) for eternity.
(This one feels sooo much like WH40K).
Now, imagine if the MC, while casting that Black Ergo spell, thought to himself that death is such an easy way out for the scumbags that he had maimed or melted in the castle, and Bel, being his other half, simply said:
"Say no more, fam. I gotchu."
Imagine what will happen to those dead or dying scumbags that got absorbed into that black orb.
Either oblivion awaits them, or they got transported to somewhere much, much worse where there are only wailing and the gnashing of teeth...
Shivers.
Well deserved for those scumbags, but still...shivers.