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I never had a dog snap on another dog but I did have a dog that went insane. Everytime we left the house the dog we destroy shit. So we took the dog to a dog therapist (not cheap). It ended up that our dog had some sort of seperation anxity when we left the house. It ended up getting so bad to the point where the dog would hurt it self destroying stuff when we were gone, so we werent really given any other option than to put it down.
But your case sounds different, I would say if there is any other incident like that take it to a dog therapist. Other than that I'm not entirely sure what it could be.
Hope that helped and best of luck.