IMO, the one huge difference between the effects of considering euthanasia for people versus animals is that people can *know* that others are considering this and animals do not. That adds the dimension of horror that we feel about placing an animal's need before a human being's need. To know that your life would be paid in order to save a cat (or even another person) strikes us as abhorrent because none of us would like to find ourselves in the position of the sacrificial lamb.
Again, the point is to treat all well according to their perception of good treatment. If I am right about animals not knowing that we talk about killing them, then we have a different obligation to them than we do to people who "get it".
But man, oh, man we have done badly by some suffering people in the name of the sanctity of human life. We have gone so far as to label suicidal people insane by virtue of their wanting to die, thus depriving them of autonomy over their own experience. Oh, and we ration health care as well... de facto agreeing that those without the ability to pay enjoy less freedom over the pursuit of life than do those who can pay.
I don't think we can call ourselves perfectly moral.