This is a bit controversial. But my boyfriends boss has just been in a similar situation. Lovely dog, his boss is a lovely bloke too, we interact with him, his wife and children too so we both know how lovely they all are and the dog had a good upbringing so far (dog was only 2). Just a couple of weeks ago his boss was mowing his front lawn and the dog was out there with him. There was a person walking past with a little Yorkshire terrier and for absolutely no reason the dog went over to the terrier and ripped it to shreds, and unfortunately it died, the only thing that stopped the dog was having a hose sprayed on it. His boss decided to take it to be euthanised straight away, as that was the only option, the police would have it destroyed or it could switch again at any time, and it wasn’t worth the risk. He’s devastated, that dog was so lovely. Anyway, moral of the story is it don’t matter how lovely a dog is when it starts switching that’s it then, they can switch any time. Regardless of how nice you are, how well trained a dog is, they can literally turn at any time!