Is assisted suicide a sin?

I heard about this story once, don't remember all the details, but a doctor's daughter was very ill, suffering tremendously, and he hated seeing her in pain. She was on the verge of death and the doctor decided to euthanize his daughter to put her out of her misery. This was a big controversy and he was put in jail for it. He truly believed he was doing the best for his daughter, by ending her suffering, and letting her rest in peace. In that situation I would have to agree with the doctor, I believe he made the right decision. This is quite a controversial subject but I believe it is okay for people who really are suffering from a terminal medical illness in which they cannot come back from.