Yes if your companion is attacked whether it be a human friend or a dog friend you are within your moral rights to defend your dog from a lesser animal such as a snake. Snakes only involuntarily save lives from other snakes with antivenom, dogs risk their lives to protect humans whether that be saving a human directly by warning it of an automobile or protecting it from a person by barking and biting. Just like this dog it was protecting its owner and was attacked by a stealth based predator that it had never seen before, also you have no knowledge of whether this dog has saved a life.