Anyone who did kill a cat forfeited their soul to the dark-side.
Although historical records indicate that cats of all colors have been associated with witches at one time or another, it is now a convention that archetypal witch's cat is entirely black. If the cat has so much as a single white hair, some authorities claim, it loses its potency as a vehicle of magic.
Witches are also supposedly often took feline shape themselves.
A black cat that crosses a person's path bestows good fortune and enables a person concerned to make a wish (though opposite is maintained in the USA, Spain, Belgium, where white and grey cats are preferred and a black cat only brings bad luck). None the less, the symbol of the black cat in the British isles at least, where simply touching such a creature is lucky and where they are common motif on good luck cards and so forth. White cats are widely distrusted throughout Europe, while stray tortoiseshell cats are most unwelcome in the home for fear that they bring bad luck with them. Cats should never be bought with money, incidentally, for doing so means they will never be good mouse-catchers.
I SHALL GO INTO A CAT,
WITH SORROW AND SIGN
AND A LITTLE BLACK SHOT.
AND I SHALL GO IN THE GODDESS'S NAME
AY WHILE I COME HOME AGAIN.