I work in a veterinary hospital and see this far too often. Poisoned food in dog parks, poisoning the neighbours cat, its wrong. Plain and simple, no animal deserves to suffer like that. (Have you SEEN the effects of poisoning??? It's enough to make anyone cry & have nightmares.) There are cat repellants out there that work if you are that fed up with a cat in your garden. Talk to the owner if it's that much of a problem. Have a hose? Use it, it's only water, but for pete's sake no animals deserves harm to be done to it just for being what it is.
Rebecca - please send my best wishes of getting well to Spook and your sister. I am also in B.C. and am curious what area this happened in. If you could PM me or email me @ email@example.com
with the location I would appreciate it.