So my Argonian was out wondering down around Riften cause that is what he does. I ran across a lodge with some blokes calling themselves Black-Briar mercenaries, now I am a peaceful fellow but when you raise sword against me I get vindictive. So after a pitched battle they all lay at my feet. I went in to the lodge to see if I could see what was what. While I was in there searching I turn and see an imperial who takes off. By the time I was out the door he was gone down the road.
I am wondering if anyone has knowledge of these blokes. Did I perhaps get on the bad side of the Maven and what might the consequences of that be I wonder?

There is a side-quest in Riften, in the inn, where you're asked to steal a horse from there. The enemies are always hostile, quest or not, but they're there to protect the horse.

Yeah, I was forced to waste those dudes as well. I didn't notice any imperial, though, he must've slipped out before I could see him.

The Imperial had nothing to do with the quest. He was just an Imperial messenger.

I get the Imperial Messenger bug A LOT. He'll just show up and take off running. I caught him once and he attacked me.

^this, I have never had a courier actually give me a quest or note, they just run away (one of them was naked one time...)

Interesting yah most couriers I have run across wear general clothing except that one chap that runs around in his loin clothe. He has got to be cold maybe that is why he always runs away after talking to me. This guy was in full legionaire light armor.

Probably you've got in their way while they were protecting the horse and took out your weapon so they attacked you