I honestly feel it depends on the situation. If the person is dieing of lung cancer or some other type of cancer where you suffer; then I think it should be up to the person that says weather they want to live or not.
The pain people endure is so terrible that if they know they are going to die for sure, it should be their decision along with one other person. Just like if you become brain dead there is always a person that decides weather or not to pull the plug.
Like vz71 wrote, it’s not our life to give or take. Nonetheless, Scott6003 brings up some interesting points.
It would seem easy to “judge” others in pain who might want to die just to end the suffering. That is certainly understandable, from both sides; on one hand, when we reject our own life, it’s offensive to God, on the other, one cannot really put oneself in another’s shoes who is experiencing chronic, severe, unrelenting pain and say, “just suck it up”.
Now, “brain dead” is a different story, in a moral sense. Assuming by brain dead we mean being kept alive by a ventilator, or other “extrodinary means”, it is licit to allow death by "pulling the plug(s). Fortunately, though, brain dead humans don’t feel pain.
Where moral decisions become more difficult, however, is in debilitating, painful conditions like with terminal cancer and in cases like Terry Schiavo, where she could live with nutritional and other supportive care.
There is also the psycological dimention, where some people could live out their terminal condition with proper degrees of pain control, if they’re so suited, and others who could be easily talked into so-called “assisted suicide” by the Kevorkians of this world, when they could live out a relatively comfortable life with proper medical care.
The most difficult cases are where the pain is so severe and unrelenting, the patient would have to be drugged into a stupor in order to be comfortable.