I don't have an answer to your main question (I was fairly sure that killing cats was an early modern thing, based off a famous book by Robert Darnton which I've never read, but I don't know enough about it to be sure), but I can tell you that this:

one of the reasons that Poland was largely avoided by the Black Death was the fact, that the Poles didn't kill cats, so there were fewer rats to spread the disease.

is most likely rubbish. There is ongoing debate about the biological nature of the disease which caused the Black Death, but approaches which combine historical, archaeological and bio-medical approaches seem to be very slowly coming to a consensus that it was a now-extinct variant of the bubonic plague (Yrsina Pestis) which was spread person-to-person, probably airborne. The abundance or absence of rats almost certainly had nothing to do with the mortality rates due to plague in the 14th century.

In A Realm Divided: A Year in the Life of Plantagenet England, Dan Jones mentions that cats with attractive coats were killed for their fur. I don't have the book with me since I'm at work, but I'll find and post the passage later tonight.

EDIT: Here we go.

Cats were not regarded as human food - the fact that they ate other animals made them as taboo as carrion-feeding birds - but cat fur was a cheap luxury, which social convention permitted the lower orders to wear. Good-looking cats were very often caught and killed for their skins.