One thing for sure now, this Meliodas seems enjoying beating his foes ! Those evil-like smirks is not something the Meliodas we knew used to display ! His demon mark is also closer to the one he had when he was the heartless and evil demon that both Estarossa and DK talked about ! He seems a lot more powerful too given how he disposed quickly of both Monspiet and Derrierie !
Will Evil Meliodas ( or Meliodas possessed by the DK ?) be the final antgonist of this arc ?
Theory time!
-At some point in the future Meliodas is going to be fighting Zeldris (Estarossa may be dead by this point, rest in peace yandere-chan)
-He realizes he can't beat Zeldris without unleashing more of his power, unexpectedly unleashes the demon king
-Zeldris is pissed, he doesn't want the demon king revived (daddy issues? daddy issues)
-Zeldris fights !possessedMel. and uses his God magic to somehow get the DK out of him
-Zeldris, Meliodas and Estarossa (if he's still alive) fight the demon king (thus fulfilling the actual Arthurian story, where the king is killed by his three sons)
-Zeldris and Estarossa actually die (or are incapacitated)
-The demon king is finally excised from Meliodas, allowing him to use his full power without risk of going all crazy
-Unfortunately, this triggers the goddess clan, who finally show up to deal with the demons once and for all
I don't know what happens after. My conclusion though is that Mel can only use his full power without going crazy if he gets rid of the DK, so the DK, would have to be the final villain. I want to see evil Mel though.