This may be a wild guess but I think it would be awesome of king Arthur was actually one of the seven deadly sins

I dont think he is.... there are only two sins left, one being Merlin and the last one having the letters "AN" in the name for sure (maybe even a C, we dont know much about him yet though iirc).
Though its not farfetched to assume the hooded person who is with Athur to be Merlin....well, for obvious reasons 😛