Originally Posted by Elli

So it's not just William, it's his dog too.

William is doing the absolute best thing by ignoring Killer. Maybe William could start bringing treats in his pockets whenever he crosses through the yard. Sit still somewhere neutral and let Killer approach. Treat on approach, even if he has the toss the treat in the dog's direction. Walk away casually. Just try this many, many times but without William's dog in the picture. Put the dog inside or somewhere else where he can't interfere or get protective.

Good luck!