One of the most difficult behaviors to change is one where your dog has already started killing other animals. The two types of killing are accidental and predatory.
Accidental killing occurs when a dog is innocently acting out his curiosity by romping with, or throwing about, a small animal such as a kitten, a gerbil, or a mole. He is usually trying to get the small animal to play.
Dogs who kill by acting out their predatory instincts usually do not eat their prey. They stalk, lie in wait, and attack the animal at the base of the neck. These dogs are more difficult to correct.
Many dogs who kill have a history of too little activity with their owners. Also, some owners make the mistake of "siccing" their dog on stray animals, like squirrels, who enter their territory. This is interpreted by these dogs as an "okay" signal to attack and kill. Many animal-killing dogs are leader types who boss their caregivers and have taken over the house and yard.
The first step in correcting this problem is to establish firmly your leadership role with your dog. This can be done through obedience training or by the following method: You must ignore him completely, except to feed and let him go outside to the bathroom. Cut off all play and petting, and even avoid eye contact. Do all of this for at least three or four days or until he is craving attention.
The next step is to show affection only after he has responded to some instruction from you. This can be a simple "sit," "lie down," "stay," or "come." When you feel confident that you have established your leadership, borrow a friend's cat, a rabbit, or chicken, and teach your dog how to behave around it. You become the dog's emotional leader by acting happy at the appearance of the other animal. Before you do this, however, consider the safety of the other animal by putting your dog on leash and enclosing the "prey" animal inside a protective barrier such as a cage. Set it up so that you are in control.
When your dog notices the other animal, act happy, give him a toy to play with. In other words, keep him emotionally involved with you. Whenever he pays attention to you , rather than the other animal, give him tons of praise- "GOOD DOG, GOOD DOG!"
Continue with this routine for several days until the mere appearance of the "prey" animal causes him to turn his attention to you, rather than toward it. Bring the two closer and closer together, still in a controlled setting, until your dog appears to accept the other animal. Ease up on the control and test your dog. If he responds by paying no attention to the animal and by giving you all of his attention, discontinue the training.
I would urge you to seek a professional animal behaviorist to help you work through this procedure with your dog.