Mercy Killing - A compassionate step or glorified murder

"Coma patient dies after Australian court rules to remove life support"

Today's headline in CNN.
So the Judge acted God and decided that the poor victim of an accident is no longer needed in this world. Despite his family's denial, they lost legal battle with the Hospital and the hospital cold bloodedly removed the life support. The reports say the patient was still breathing for some time. But finally he gave up on life.

I have had several discussions with my friend Karthik on this issue of Mercy Killing. Is it ethical to allow someone to take someone's life? Though this was not a clear case of mercy killing as the patient himself did not consent. But the question is, if a terminally ill patient says he wants to end his life, shouldn't he be allowed to act as per his will? Or many other lives also depend on his life, emotionally or materially, and so he should not have a say in whether to end his own life or not.

Is it fair to kill oneself? If yes, in what circumstances? If no, why not?