I'm pretty sure this is very common...if you happen to piss the right people off.

However, I haven't murdered anyone or anything...as far as I know. I am in the thieves guild yes, but never hurt anyone. Unless they attack me first.

However, an assassin from the dark brotherhood just tried to kill me...he was carrying an assassination letter to kill me.

Can I still join the brotherhood though? As that is my aim with this character.

Just a random encounter. Nothing initiates it. And yes, you can still join the Dark Brotherhood.

Random encounter. I've given five or six assassins an axe to the face lately.

I think this is hard coded into the game. Every one of my PCs (which is only a few, but happens in every case) has run into this so I don't think it matters what you've done or who you've killed. The first time, I was so worried about the added threat of assassination that I promptly started the Dark Brotherhood quest and wiped them all out once I had access to their sanctuary. I realized this was something of an over-reaction on my part so I just started ignoring them in later play throughs. The assassination attempts continue but it's no problem (mostly) to kill the assassins. It's too bad you don't get any serious loot off the assassins.

Thanks all

They stop attacking once you join them... I RP abit and view as a "test", where they're watching me to see if I'm truly as skilled with a sword in hand as I'm known to be