There is a Vulture interview with Ed Harris where he expresses disappointment at where his character has gone the last season or two. He says he signed up to play the Man in Black, not William the insane old man. But it looks like he's getting to be the Man in Black again. I watched that post-credits scene again last night and you can see how much fun he's having as he's kicking his own ass.
William: "Who are you? Are you her, too?"
Both: "You may look like me, but you don't know me. You're just some fucking cheap imitation."
Hale: "Do you know how easy it is, William? How little it takes to know you?"
Man in Black Host: "That part of you that indulged in bloodthirst and savagery at the park, you always thought that was the darker side of you. A stain to exorcise, to scrub out, one month of the year at Westworld. But there are no sides. That was you. Now... that is me. *slice* ... Welcome to the end, William."
Season 4's gonna be a doozy.