This is really weird, but I seemed to have fought all six of the brothers before finding the tunnel. After I killed the last brother, I went around all the barrow mounds again and finally found the tunnel in Karil's tomb, but I already have a kill count of 6. Has this happened to any of you?

Yeah it happens occaisionally. If you did a run previously and half killed a monster there, then opened the chest and left, then another player finishes off that monster, you will get the killcount for it. Quite a rare occurence because you have to get rght out of the tunnel so it stops shaking for your kc to reset.

Oh, ok, that makes sense, I must've thought I had already killed Karil