My dogs used to play tug-of-war with cats that came in the yard if they could catch them. You could hear the cats bones breaking and crunching as the dogs played. Its just the nature of dogs to go after cats unless they have been raised with cats around.
Either way they'll make more dogs and they'll make more cats. I wouldn't worry about it much unless the dog goes after people.
"If ye love wealth greater than liberty, the tranquility of servitude greater than the animating contest for freedom, go home from us in peace. We seek not your counsel, nor your arms. Crouch down and lick the hand that feeds you; May your chains set lightly upon you, and may posterity forget that ye were our countrymen."
Samuel Adams
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