Snape's worst crime

I'm re-reading the half blood prince.

There's the old is Snape evil debate... bully to the kids, etc.

I think his worst crime was in his teaching. I always thought he was a poor teacher, but he wrote all the notes in Harry's copy of the textbook and never bothered to share this knowledge with his students. And this was stuff he learned when he was on a teenager; I assume he knew more later.