I think this section is a bit of a red herring. Slughorn's a bit of a narcissist - perhaps not someone who formerly suffers from the disorder, but certainly someone who only wants to associate with the brightest and best (thus his special invitation-only parties). But like many narcissists, he has a deluded sense of his own importance, and how he appears to others; in this sentence he focuses on himself and his relationship with Snape - "I taught him - I thought I knew him" - whereas other, less self-absorbed people might just focus on Snape's apparent crimes - e.g. "How could he do something so awful"?

I think he is shocked and shaken not because he assumes teaching / knowing someone means he can predict who will and won't be a murderer, but because Snape's act undermines his believed understanding of Snape's character, and also suggests that he may have gone wrong somewhere, as Snape's teacher.

I think this section is a bit of a red herring. Slughorn's a bit of a narcissist - perhaps not someone who formally suffers from the disorder, but certainly someone who only wants to associate with the brightest and best (thus his special invitation-only parties). But, like many narcissists, he has a deluded sense of his own importance, and how he appears to others; in this sentence he focuses on himself and his relationship with Snape - "I taught him - I thought I knew him" - whereas other, less self-absorbed people might just focus on Snape's apparent crimes - e.g. "How could he do something so awful?"

I think he is shocked and shaken not because he assumes teaching / knowing someone means he can predict who will and won't be a murderer, but because Snape's act undermines his believed understanding of Snape's character, and also suggests that he may have gone wrong somewhere, as Snape's teacher.