Is Buster and outside only dog? I've heard of dogs that will accept cats that live in the house with them but will chase cats outside of the house. If he is an outside only dog I would try to house train him and adjust him to having manners indoors first then slowly introduce the cats (with him under control, on a leash or something). My siberian husky is a lot calmer and more well-behaved in the house, outside time is running and playing time! I wouldn't ever leave him alone with any cat, personally either especially if he has killed a cat before. Supervised interactions only, and don't ever let him get away with chasing a cat, period!