Theories on William?

He was just trying to cover his tracks. He probably had some sort of history of manipulating people in this way. To me it seemed like he either had Antisocial personality disorder (Sociopath) or Narcisstistic personality disorder. I'm not sure if he just got off on manipulating and runing people or he somehow feels other people deserve it.

The man who can give himself a hug everyday, truly has the arms of a Gibbon

I think he was trying to be good--but his interaction with his dad was what finally put him over the edge.

I also agree with the two people above me. After he had kissed his mum on the cheek, she gave him a hug, and whispered in his ears, that she loved him. I believe he wanted to stop because of that. For that split moment he was happy and didn't want anyone hurt. But his father killed it all and pushed him to the edge after the fight, when all he was trying to do was stop Jim. After he had pushed his father down, the first thing he did was look at his mum, to see her expression. She seemed disappointed and that feeling of his mum loving him went away. Which resulted him to go back to get Jim to commit suicide.

Neither, he was a psychopath.

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Sir Ian McKellen? That dude must be knee-deep in boob.