I'm so sorry to hear about this happening, but I 100% agree w/Glacier. Definately fix your fence asap. Your dog isn't a bad dog for acting on instinct. Nikka my pointer X has killed a gopher that was in our yard and she's a cream puff. My RB terrier X, Sandy was the sweetest guy around, he loved everyone, dogs, cats, kids, people, whatever. In fact one of his main playmates was a rabbit when he was young. But my brother's hamster got out of it's cage once and Sandy killed him without a second thought before we realized what was going on. It's hard to fight a dogs prey instinct, I'm not even sure it's possible, maybe one of the more experienced people here might know, but you can work to prevent putting the dog in those situations.