The question of "Old Norse Murder Mysteries" I find rather droll - written no doubt by Agata Christensdottir  Ten Little Indians would come out as Ten Little Thralls  and "Murder she Said!"  might be "Morðvig Sagði Hon" yes  I think I like the idea
 
 
> Hann mælti: "E.g. vil að hann segi mér hvar hann hefir hulið hræ Atla.
Mér ER
sagt að hann hafi illa um búið."
> He replied "I want him to say to me where he has hidden Atli's body.
It is said to me that he had done (this ) badly
 
I see him as a "Man on a Mission" - (play "Godfather" theme here,) and I assumed that since Bergthora had given him a 'contract'  on Brynjolf  then the question of the disposal of Atli's corpse was merely an excuse. He had been given this job by the Godmother and it was an offer he could not refuse.
 
 
 hversu mér bregður við"
> how it will move (affect?) me"

"how I'll react."