Is euthanasia ethical or unethical ?

is euthanasia ethical in pets that can't be saved. no one wants to see a pet suffer that's why we take our pet to the vet when we see it suffer. so that the pet can be put out of its misery.now this is seen as o.k. because the animal is suffering. when you help a human that is terminally ill commit suicide, it's murder. why do we view taking the animals life as ethical and letting the human suffer as ethical ?