It was really weak and lazy to plot Zeldris becoming the demon king, Meliodas just assumed that Zeldris was killed by the demon king, he didn't even try to save him after the fight, didn't even check his body or give him a burial, he just left Zeldris where he was, released Gelda and said for sure that Zeldris wasn't in this world anymore, but he wasn't even sure of that or saw it happen, it made him look like a bad brother after all the talk with Ban in purgatory, all his regrets about not being a good brother to Zeldris, does he just do that? Seriously, what the hell was that Nakaba? Furthermore, it was supposed to take days for the commandments to be absorbed, but Zeldris simply absorbs them immediately.