So i tried to see how long i could survive by going on a rampage around whiterun, after killing almost all people from the city, the courier comes running to me and gives me a letter asking me to come talk to him to do a certain service, for having killed brenuin (Jervar is the son of the owner of the stables in whiterun, and Brenuin is the bum that lives in the city).

The letter praised my "use of power when needed" and asked me to go meet him, i did so after losing my bounty, and he didn't give any new conversation options, maybe because i killed almost everyone in the city? Or maybe because the letter dosn't really change anything... anyone seen that happen?

He won't have any new dialogue options. He says what you need to know when you first approach him, outside of a dialogue box. Chances are, if you just ran up and talked to him, you might've missed it. He wanted you to go beat someone up.

I see, i got curious to know what was that all about, but i'm not really going to do it... hate to do bad stuff on normal playtrough, only if that person really deserves it ._.