I would call a trainer or someone as soon as you can but make sure to keep them seperated until. He is a hunting dog if im correct so his prey drive is very high, im sure he didnt try to hurt the kitty but saw something running and attacked it by the time he was done it was prolly already to late for him to understand it was his friend. Im sure a trainer could help you guys out or atleast hope so... Sorry about your loss it mist be very hard to look at duke and not cry right now as well.