So considering the timeline of the event, what was Diablo's trap? Was it simply fueling Imperius's anger and creating a rift between the Council, or was it something else?
It could explain Imperius's kill vote after the Sin War, which Diablo would not have liked, but he didn't know that the whole Sanctuary thing was going to happen. It could also explain Malthael's change in personality.
Deckard Cain:I have heard tales of Malthael staring into the chalice for years on end in his quarter of the High Heavens, the Pools of Wisdom. The pools are infinitely deep wells of emotion. Those who gaze into them see not their own reflection, but the sum total of emotion that all sentient beings in the universe are experiencing at that moment. It is from these pools, I believe, that Malthael draws to fill his chalice. One intriguing fact concerning Malthael is that the darkening of his mood is said to have taken place in the years following Sanctuary's creation. ... Perhaps Malthael's chance in demeanor is connected to his chalice and the visions he beheld there?
P.S. Yes, Valor turns to Wrath blah blah blah it's quite obvious.
Edit: I should learn to read. The description of the video answers my question.
Long before the creation of Sanctuary, the Eternal Conflict between angels and demons raged on for untold millennia. Blizzard Entertainment teamed up with renowned director Peter Chung and acclaimed animation studio Titmouse to create this unique vision of a fundamental moment in the battle between the High Heavens and the Burning Hells.