The thing that took place is really unfortunate but you should not be putting the entire blame on the dog. May be it did not have the intention of killing it; it was just an accident. But what is more important to you is to understand the various moods and behavior of your dog. You can get in touch with someone who is writing a paper on this particular topic only- the various mood swings of the animals, just like we go through. Even they feel isolated and lonely and that is the time when you must be just beside them, caring for them, loving them and adoring them. Even they need to be felt special.