Don't fret.Nature intends rabbits to be killed by other animals.That's why rabbits breed so fast,run so fast, have a warning white scut, have excellent hearing (hence the big ears to catch the sound) and dig burrows to shelter and sleep in.Evolution has meant that the species has the means to avoid being wiped out.Conversely, if there were no predators there'd be so many rabbits that the rabbits would starve.
Your cocker spaniel would kill an animal if she had the chance and the animal triggered her instinctive hunt and kill behaviour.Dogs are not vegetarians !
But , seeing it happen can take a bit of getting used to.