I think he's angry with Delores because he was in love with her but realised that she couldn't return the love and the entire experience changed him.

@LadyGigi said:

I think he's angry with Delores because he was in love with her but realised that she couldn't return the love and the entire experience changed him.

He was also obsessed with the maze, and didn't realize it was about the hosts, and it wasn't about him.

@nemo69 said:

@LadyGigi said:

I think he's angry with Delores because he was in love with her but realised that she couldn't return the love and the entire experience changed him.

He was also obsessed with the maze, and didn't realize it was about the hosts, and it wasn't about him.

I saw William as a jilted lover. I thought he wanted to know about the maze because Delores was so preoccupied with finding the center and he couldn't take her attention away from it. Apparently for years she went on and on about the damn maze yet couldn't find the center. So he got frustrated with her obsession with it so he decided to find it himself. I think eventually he just started resenting everything about the damn park but was still addicted to it.

@Teapot2k said:

I think I understand "most" of the show. But I still don't understand how William seemed to care for Delores, but the Man in Black is so cruel to her? When did everything change?

I think it changed when he was back at the beginning of the park and he saw her and her memory was wiped and didn't recognize him.

I see the following factors at work:

1. Delores doesn't remember him--and even though that's something he should have entirely expected, it shattered his illusions that she was more than just a robot, especially given the lengths to which he went to find her.

2. In his murderous rampage against the hosts, he discovered he had a taste for violence and cruelty.

3. If we take him at his word (and granted there are grounds to be skeptical), he isn't a bad guy in the real world; his villainous character we see is merely a role he plays while inside the park. Since the hosts are machines that don't really die, he doesn't view his attacks on them as a crime, and from all available evidence, the society he's in agrees.

4. Sort of like Jigsaw from the "Saw" movies, he seems to think his cruelty toward the hosts has a noble purpose, to provoke an awakening in them.

Note here that I'm simply explaining what I think the show was attempting to say about his character. I personally think this plot point, while clever in its execution, wasn't altogether believable or convincing. It felt kind of like a comic-book villain's origin story, where a sweet and likable character has some bad experiences that turn him toward evil, and while the show did take pains to depict the events that furthered William's transformation, it struck me as overly contrived.

@Teapot2k said:

I think I understand "most" of the show. But I still don't understand how William seemed to care for Delores, but the Man in Black is so cruel to her? When did everything change?

I think the thing he loved (Delores and Westworld) became the thing he hated because he found something real at Westworld (risk/danger?) that he had never experienced outside of Westworld (safety/self-indulgence?), but the tragic flaw was that he (or other guests) couldn't be hurt at Westworld. This is why he became so abusive to Delores, and why he was glad to be shot at the end.