I would kill the cats in some quite fashion be it pellet gun or 22 cb shells, trapping them first may be easiest , just make sure you don't talk to anyone about it and have an area where you can do it away from prying eyes. Do you have a garage? I'm not a huge fan of drowning either but you could put the cage trap in a garbage can and run a hose into the can from the exhaust of your car and gas the darned things to death, I'd think it would probably put them to sleep first then they'd die of carbon monoxide poisoning, I don't know how much pain that would cause if any.
It's hard to believe the city could prevent you from protecting your animals, shooting a gun in the city limits may be illegal but doing so to protect your property or yourself from a vicious animal should waive that petty crime. Many people conceal carry for self defense and discharge guns in cities in defense of themselves or others, I have never heard of them gettin cited for shooting in the city limits? I should think the same applies to defending your chickens and I am sure most judges should see it this way as well.
It's hard to believe the city could prevent you from protecting your animals, shooting a gun in the city limits may be illegal but doing so to protect your property or yourself from a vicious animal should waive that petty crime. Many people conceal carry for self defense and discharge guns in cities in defense of themselves or others, I have never heard of them gettin cited for shooting in the city limits? I should think the same applies to defending your chickens and I am sure most judges should see it this way as well.