I have had an elderly cat who has been killed as a result of being attacked by neighbours, and it is extremely upsetting. Although I normally would not advocate violence, anyone who can harm an animal is not worth the normal social considerations. And the law seems to do little to protect animals, especially cats. People who get pleasure from killing animals are evil. However, having said that, you should contact the police to let them know, if you know who is killing kittens.