Ok, it's difficult to imagine how Sanguinius was killed. No one in all 40k fluff was killed more thoroughly and painfully than he. Horus killed him on every level, physically, mentally, and spiritually. He literally ripped his soul to pieces and threw it to hungry daemons. He used chaos to give Sanguinius the most painful death possible by his means. As Sanguinius' blood boiled and his soul was ripped apart he let out a psychic scream that echoed in the heads of every BA. Henceforth every single individual that has Sanguinius' blood in their veins suffers from images of his death (a.k.a the black rage). You could say that the images and feedback from his death is contained in every drop, in every cell of Sanguinius' blood, blood that runs in the Blood Angel's veins. And the longer you have blood angel blood in your veins (i.e. the longer you go on living as a space marine of the blood angel's chapter with Sanguinius' gene seed) the more chance you have to become susceptible to the black rage. Eventually, every blood angel will experience it, one way or another.
Oh, and according to Inquisitor fluff, the Emperor is already "insane". He seems to have multiple personalities who disagree and speak of their own accord. I can't blame him though, 10,000 years of hearing/feeling thousands of psyker deaths run through him every day... That and trying to maintain his grip on life and to do what he can for his failing Imperium.