While mine have never killed kittens or cats, I have seen them come close for two reasons:

1)The cat ran, and they kicked into "prey drive" instinct (and this is after living with this particular cat their whole lives )

2) Star lOVES cats and was playing with my son's kitten, who also adored her. But Star got a little too rough and I thought she would accidently kill the kitten while trying to have fun - so no more kitten playtimes.

Is it possible the lab was trying to play with the kittens?