in the uk some shit bags poison cats with (well id rather not say but it goes in your car) and thats the way they kill em it takes around 24-48hrs and they get kidney failure, a very aweful way to die if you ask me. i don't know why they do it, they are just scum and deserve to have it done to themselves, it's not just our planet !
anyway if your cat had been poisoned you'd know , it would be vomiting at least a few times before hand and its not normally quick.....
hope this helps
sorry to here this story, its really sad :(