Why was Slughorn so surprised that someone he taught was a murderer?

'Snape!' ejaculated Slughorn, who looked the most shaken, pale and sweating. 'Snape! I taught him! I thought I knew him!' (Half-Blood Prince, Chapter 29)

Considering the long and illustrious list of future mass murders that Slughorn taught, knew, and was quite friendly with:

• Tom Riddle, aka Lord Voldemort


• Death Eaters (We see Lestrange and Avery in Slughorn's memory, socializing with him among other Riddle's friends all of whom became Death Eaters later)

... what would cause Slughorn to somehow assume that teaching and having knowing someone would be effective at figuring out they aren't a killer?