They have no natural preditors and are devestating to birds and other wildlife. These are NOT the cute, loving kittens that people think they are. You'll save many a bird's life by destroying these destructive creatures. Cats have no morals. Feral cats are no more than someone's pet that has been deserted and left to fend for itself.
Birds are just one item in their diet.
While my cats are indoor only because I feel that is best for both them and the birds, killing an animal simply because a human did not take the committment of pet ownership seriously is absolutely not an option. The attitude of anyone whose first reaction to some type life they don't like is to kill it has a very narrow view of the world. He understands that cats are doing terrible things to our bird species. I have a friend with 5 feral cats. Carle is right.