was a failed experiment. His mother, Misako Togane, wanted to create an artificial criminally asymptomatic person. As an experiment Sakuya did show very good positive results, such as when he demonstrated sadistic acts towards defenseless pups repeatedly, and that having no negative impact on his Psycho pass. Similar conclusions reached from repeated observations of Sakuya's acts led his mother to prematurely conclude that Sakuya turned out to be a successful experiment, and that as a criminally asymptomatic person Sakuya is worthy of joining Sibyl system, much like Misako herself.
However, Sakuya's failure as an experiment came to light when he failed to kill his mother, and her brain was scooped out of her body much to his horror. He tried to kill his mother because he could not stand the thought that her mother was leaving him for something/somebody else (in this case, she voluntarily agreed to be subsumed by Sibyl system). He was fixated on this thought that if he could not have his mother, then nobody else should be able to either. In order to ensure that he attempted to murder his mother but failed eventually.
The combination of his stark failure to achieve the desirable outcome and the consequence of his failure he witnessed that ultimately darkened his hue to pitch black. This pitch black hue, an extreme end of hue spectrum, was not recorded for any other human being in the Psycho pass universe that existed up to Season 2. So whether the related crime coefficient is shown to be 799 or 2000 or any number, it is not the point of focus. It is an arbitrary number to show the audience something that should stand out among all the recorded Psycho passes. The focus should be on the pitch black hue he ended up with, as opposed to clear white hue he was supposed to have as a successful experiment.
To make things clear, Sakuya turned out to be an exact opposite of a criminally asymptomatic person. And his mother was not murdered.