I am doing the main quest, and after I put the ring in the poor merchants body, the guy in the Riften Marketplace tells of another quest to do, but I end up going into the sewers to seek out the former Blade so that I could continue the main quest. I thought could join the Thieves' Guild later.

I descend into the sewers, and start attacking the inhabitants within. I thought it was odd that many of the NPCs in there were named - but then I realized that they were Thieves' Guild members!

Oh well. Reloading from an old save...

There's named bandits in those sewers, you know.

The Thieve's Guild hangs out at the Ragged Flagon, which is a tavern in one of the deepest parts of the sewers.

So as long as you haven't killed anybody there, you'll be fine.

LOL