The name is fine.
It was just weird to hear Zoro call himself that. I get why since he has to not only understand Enma's needs but to also conquer and bend the blade to his will... I get it, I get that it's supposed to be a cool line.
If other characters or even his bounty post give him an epithet similar then sure. I just don't see why someone will call him the king of hell, unless Oda is going to make Zoro have this demonic aura that gives off the vibe that he's literally the king of hell. Calling him the king of hell otherwise would mean they'd have to know of Enma's origin since it was a focus and I just don't see it. As long as zoro doesn't go around calling himself that to other people then it's fine.
So in short:
Name is cool, but could be better because OP if full of cool epithets.