The character of Severus Snape was inspired by J. K. Rowling’s high school chemistry teacher, John Nettleship. When he found out, he was shocked, and said, “I knew I was a strict teacher, but I didn’t know I was that bad.”
(To be fair, J. K. Rowling specified that Snape was actually based on a composite of two or three different people; but she took her main inspiration from him.)
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I always wonder if this is the reason JKR is generally easier on Snape than the fandom is: not so much out of respect for him but respect for John Nettleship (who is dead now.)