That's a tough question. Sure, we do it to animals, but humans is a different story. I'm on the borderline of deciding if it's right or not. I'm sure that just as long as the patient is terminal, it's an act of mercy killing. However, if there's signs the patient is beginning to recover, but a doctor puts the person to sleep anyway, it could be called murder. It's considered murder if a loved one pulls the plug to put them out of there misery, but it's mainly because they don't want them to be in anymore pain.
It's still debateable, really, if euthanasia is right or not. If the person or person's loved ones wanted it, then it may not be considered wrong. To be honest, I'm not very sure, but I'll say I agree with it to a certain degree until I know more about it.