The law in the USA is that it is perfectly legal to destroy any animal, someone's pet or not, that is threatening the health, well-being, and safety of yourself, your family, your animals, or even your property. Also true even in most densely populated cities, firearms laws permitting, if not then 700-1200fps air-rifles are commonly used. The only animals exempt from you taking immediate action, legally, are those listed on endangered or threatened species lists, and any bird species under protection of MBTA (the Migratory Bird Treaty Act). Even then variances can be given should there be sufficient problem but this requires further study by authorities. Since cats are listed in the TOP 40 WORST invasive-species of the world in the "Global Invasive Species Database", this means they have no protection whatsoever from being shot on sight, they are not on any protected-species list anywhere in the world. Quite the opposite as a matter of fact. And if your area enforces and obeys invasive-species laws -- as they should -- then it is against the law to NOT destroy any cat on sight, someone's pet or not. It is your civic and moral responsibility to destroy any invasive-species that is found away from supervised confinement and roaming freely in a non-native habitat.
Shoot to maim is punishable under the laws that define animal-cruelty. But shoot to kill is a perfectly legal way to humanely destroy an animal. The same laws that apply to methods of humanely hunting animals also applies to cats. Unlike cat-lovers' psychotic beliefs, the reality is that a cat is just another animal. It's NOT their baby, their child, their offspring. Even if they do view their cats that way, letting them roam free is no less criminally irresponsible than them telling their child to go play in the freeway and then blaming the cars for their child's death. If they let their cat roam free, NO MATTER HOW IT DIES, that is THEIR fault and they can be charged with all laws that clearly define animal-neglect, animal-abandonment, and animal-endangerment.
In fact, here's a publication from a study done by the University of Nebraska on the best ways to HUMANELY deal with a feral-cat problem wherever you live. This documentation INCLUDES the best firearms, ammo, and air-rifles required to HUMANELY destroy cats. deenawinter.files.wordpress.com/2010/12/ec178...
Besides, what difference does it make if the cat gets shot or ran over by a car, attacked by another cat or animal, drowned, or poisoned by plants animals or chemicals (inexpensive 1-adult-strength generic acetaminophen pain-relievers gaining in popularity, for being so species specific). The result is the same. The cause is the same -- the fault of the criminally irresponsible pet-owner that let their invasive-species pet roam free. They've already proved that their animal is 100% expendable. You can either destroy their cat for them humanely, or let their lack of care cause it to inevitably die inhumanely. They don't care one bit how their cat might cruelly suffer to death if they let it roam free. Humanely destroy their cat for them before that can happen.
Case in point: As in this very instance -- humanely shoot their cat for them before some other neighbor of theirs gets so fed-up with their cat that they lop it in half with a machete.
This isn't rocket-surgery folks!