I finished the series about a week ago.
At first I had trouble buying the William/Man in Black thing. The Man in Black character was too much of a departure from the William personality. Kind of like: if you freeze water, you'll end up with frozen water- you won't get frozen peas (or any number of other things that can be frozen that aren't water....unfathomable hurt/anger here is the "freezing" process). The Man in Black came across as a bona fide psychopath to the core, and William came across as someone who couldn't help but empathize with machines in spite of logically 'knowing' they were machines, and didn't seem to want to turn that empathy off. Of all the human characters in the series, he dehumanized the hosts the least. I was listening to some podcast recently discussing the show, in which the person opined (or rather, quoted someone else having this opinion, but he agreed with it- as do I) that even if we aren't actually hurting AI humanoids when we treat them badly- we are compromising our own humanity/character by doing it. This is something that I think the character William would completely agree with, and so I sorta didn't buy the transition.
But after I had a chance to digest it a bit, it became a bit more believable. Jimmi Simpson's expression when Delores didn't recognize him, at the end, was priceless- like he couldn't believe he'd just been toyed with like that. He basically got completely emotionally fucked over for not dehumanizing them. I can see how maybe rampantly dehumanizing the hosts was a manifestation of that anger, rather than psychopathy. And it's like, that time spent with Delores in a fictional world seemed to 'hook' him in some way that he kept coming back to understand. I really liked (the second time watching it) the conversation he has with Ford about seeking "meaning" in the maze. It's like he felt something in the fictional world with Delores that he hasn't been able to feel in the real world- and though, in his mind, the hosts are completely dehumanized at this point- he still thinks there's deeper 'meaning' to be found in the maze (and ironically, I think the dehumanization of the hosts takes him further from the 'center' of the maze).
With that in mind, I think it's a really interesting way to remake the 'Man in Black' from the original movie. I think Maeve is probably the closest literal parallel to the original Man in Black (Yule Brenner's character), but having a human who has been damaged/broken by the realistic-ness of the fictional world as the new Man in Black is a really interesting spin.