I did a search for this here and on the wiki:

I've just completed the Bedlam Quest and the Burglary Job for Vex and Delvin, sold some goods to Tonilla, and now as I'm leaving Riften I'm attacked by two (2nd reload 3) big brutes who slaughter me and the city guards do nothing.... They said, "We're here to teach you a lesson" and then as they're walking away from my dead body they said, "That's the last of them."

What did I do???

Run Forest, Run!

Really, if you can't beat the Thugs your best option is to run to another Hold.

Hired thugs, you upset someone with your antics, if you manage to survive the next encounter, check the bodies for a note.

Its most likely Mjoll sending people after you since she doesn't like the Thieves Guild. This happened to me during one of the later quests and they were actually quite hard to kill.

Great. My little hunting bow/steel dagger Bosmer will do the best she can.

Note to Self
1. Abandon Theives Guild for the forseeable future.