My wife's great grandfather killed most of the cats in the town (feral and pets)  that he lived in with poison, as he was a bird lover and could see the damage they were doing.  This early last century.

He purchased a large block of native bushland and donated it to the crown, the reserve is named after him. (not "cat murderer reserve" BTW)

I don't mind cats myself - and they're probably a minor part of the overall destruction wreaked on NZ birdlife.  I witnessed a feral boar munching down on baby seagulls a few weeks ago, as if stoats, cats, dogs, humans etc weren't bad enough.

One of the saddest things I learned was that there used to be flocks of thousands of kakariki in Chch, every last one poisoned deliberately as they plundered grain and vege gardens.

A problem with fleas and cats is that cats lack an enzyme that breaks down pyrethrum/pyrethroids, so relatively "safe" insecticides to humans and most mammals can be very toxic to cats - ie - large quantities of fly spray.