I'm so sorry for your loss. I've been in the exact same place. I just don't know which dog actually killed Dominique, who they had lived with for over five years.

Duke was acting on instinct. He is not aggressive, he is not mean, he's not a bad dog. He's just a dog doing what dogs do. He had no idea he was about to break your heart. He reacted to prey, as dogs have done for 1000s of years.

I would never let him near a cat again, especially outside. My dogs have learned the difference between inside cats and outdoor cats. An outdoor cat around here is a dead cat, doesn't matter who's cat it is, if they knew each other, if they were buddies...a cat outside is dead. Inside the cats are safe, but well supervised with the dogs. I don't allow any interaction. Dogs are kicked outside for extended staring at a cat. They are not allowed to play together, there is absolutely no chasing ever. If the dogs want to be in the house, they have to respect that it is the cats' territory.

Again, I am so terribly sorry. PM me if you want/need to talk more.