I finished Goldenglow quest on my good Orc and that's are far as I'm going because of RP reasons. However one I come back to the quest to talk to everyone they all tell me how they think I'm good etc. I talk to vex and steal a iron boot by accident, later that day a Dark Brotherhood assassin comes after me. Really why would they do this? I'm never going back there and I'm destroying that Brotherhood, that was the third assassin.
Are they supposed to do that? After I took it they said I could of just asked.

The assassins don't say who sent them. I've had people never steal anything and have been attacked by 5 assassins. They are just random it has nothing to do with the thieves guild.

The Thieves Guild aren't the ones hiring assassins to kill you. It's hard to say who sent them and why someone wants you dead but there's plenty of things that can be done in Skyrim to get assassins on your tail.

you wronged someone who now wants you dead. I doubt it was for stealing the boots. Usually you will have hired thugs come after you for theft and they will carry a letter with the name of who its from. Assassins just have orders from Astrid and never name the client.

I'm pretty sure I know at least one quest which gets the Dark Brotherhood assassins after you. I start new characters a lot because I feel too rich and powerful by the time I get to level 10, and I've noticed that on characters where I deliver Sven's fake letter from Faendal to Camilla Valerius I always get lots of DB assassins after me. However, when I just skip that quest entirely I hardly ever seem to get any. I'm almost certain that's one of the trigger for it, because if you talk to Faendal after helping Sven instead of him then he says something like "You've got a lot of nerve talking to me after everything you've done!".

Oh, so Sven is sending them after me on my game.

HE MUST DIE!!!!!