Was William about to call the whole thing off and save Jim? (spoilers)
At his mother's garden party, William seemed to have a moment of redemption when his mum told him she love him. I assume this was intentional? When he was typing furiously, I got the impression that he'd had a change of heart due to his mother finally acknowledging him and wanted to save Jim, but then his father ruined it for him... a combination of his father's words, the humilating looks from the people at the party, and his mother neglectful glance away from him at that moment made him snap, and he instantly slipped further into wanting Jim to commit suicide.
This is at least what I took from the scene. Were we supposed to assume this?
I did like this film, despite its many, many flaws, and Aaron Johnson was brilliant in it. However, I did find his character's motivations far too unclear at times; I understood that he was clearly severely unhinged due mostly to the neglectful way he was raised (in the shadow of his older brother), but he didn't seem to want Jim dead as much as he was completely obsessed with bringing someone to suicide. If the death of those around him (Jim, in particular, as he was weak and vulnerable) was his primary goal, he could easily have murdered Jim himself in the final scene, but that never even seemed to occur to him.
One day, you are going to die.