My friends' lab kills rodents. Come to think of it, I know a few. Rabbits, mice, voles, etc. They are all high prey drive field types and all rescues. I cornered a rat once -- not a good time -- and the dogs acted like we were playing a game. I doubt mine would get one. But you never know. They way they reacted to the kitten makes me believe they are not ratters. I kinda wish they were. I HATE rats!
At least he didn't bring it inside or put it in bed with you the way a cat might.