I got into a brawl with Hofgrir Horse-Crusher and after a while he started using a dagger. I continued to only use my fists and apparently I killed him, which resulted in a 1000 septim fine


If anything he attacked me first, he even got the first few hits in the brawl.

Why am I the murderer? Isn't killing NPCs okay as long as you aren't the aggressor?

Did you hit him with a weapon or spell?
Did you accidentally bash him with your shield?
If not, it`s probably a bug