I was on my way to Winterhold to join the college when an Argonian assassin jumped out of the bushes and tried to kill me. I killed her, and found a letter on her from Astrid saying somebody had hired the Dark Brotherhood to kill me, but it didn't mention who had hired them. So far I've tried to play as a virtuous character and I can't recall stealing or murdering, though I have killed a few bandits. What gives? Who would want to kill me?

That's normal. Happens in every game.

Same with Thalmor.

They like to do that..

kill him back

That's cause you made plenty of orphans when you cleaned those bandits hideouts

I think it's the witch in Riverwood... you start out on her bad side for some reason.

"As long as there's two people left on the plannet: Someone is gonna want someone dead"

Anyway: It is usually Khajiits who attack me, but yeah, it is normal.

I had a great moment when I exited a mine and was waylaid. I killed the assassin and decided for no reason of which I'm aware, to hit the "wait" button for an hour. Another assassin attacked. I kicked his can. So I waited again. Another assassin. I think this happened a total of five times. I guess they ran out of assassins.

Oh yeah... the guy who sent them after me was the one from the college who runs the library.

"Sweet mother, sweet mother; send your child unto me. For the sins of the unworthy must be baptized with blood and fear"

They're still mad because they couldn't kill the Nerevarine. They're taking out their frustration on you.