Shoot the cat in the head. It's quick. It's painless. It's humane.
Barring that, find out whose cats they are, go over there and tell
them to keep their worthless felines out of your yard. If they won't,
beat the hell outa them and then shoot the cats. You're not being
inhumane. The irresponsible pet owners are being inhumane by not
controlling their animals.

....and I hope someone controls you someday.