I think Zoro with King of Hell meant he mastered at 100% Enma ( Enma= King of Hell in Buddhism), at that time Zoro needs to unleash all his haki to tame the sword and he didn’t notice he was releasing Coc. Now he is learning how to use adCoc at his will, he already tamed Enma. The only problem is why he said “ I’ll become the King of Hell”, this is why it seems stupid this plot. I’m curious to see if his next attack is a King of Hell one or only a three sword style attack with adcoc