The bounty would have been for attacking an innocent (in this case the guard himself). There's an option in the game settings to disable attacking innocents, so you can avoid having this happen again - your attacks and abilities simply won't affect guards and NPCs in towns with white outlines.

Now as far as knowing how much gold he took... Well, I'm not sure if there's any way to tell after the fact. Before he killed you there should have been an indicator on your screen that showed how much of a bounty you had, and that's the amount that would have been taken. I don't think there's a log you can check later though. If you ended up with 0 gold left, you probably still have an outstanding bounty, though, so be careful.