The killer is the player who dealt the most damage to the victim in the 10 seconds preceding their death. That may not be the person who is displayed on the kill message in-game. In this case, this kill would have registered as an assist for Contt. You can check this in-game using /siegestats.

oh that is actually really amazing, but in my opinion the person who got the last hit should get the kill stat in /siegestats