Some have posted, "It's nature, don't interfere"; others
advocate an interventionist approach. Both make valid points.
However, in this particular case, there's not much of a choice. By
the time you realize what's going on, the dog has already maimed
the kitten beyond any chance of survival. No point chasing away the
dog or interfering, when the kitten is already as good as dead. The
kitten wouldn't be able to survive a big dog's bite.
So there's no point in attacking, chasing away, or even killing (as
some suggested) the dog. It won't bring back the kitten or help the
situation. There's no guarantee that the dog will (or wont) do this
again, so you can't confidently choose any particular course of
action.
In any situation, it's important to act out of cosmoethics, see how
you can best help everyone involved. And not act out of anger or
revenge. In this particular case scenario, most probably there's
nothing much else for any person around to do, other than to wish
the soul of the kitten well (animals have souls too, you know this
right?).