I watched a TV programme some time ago, which featured that Dignitas clinic in Zurich, where it followed the story of one lady, who was in her fifties and had an incurable degenerative disease. She wished, therefore, to die with dignity before her mental facilities failed her. The staff at that particular clinic showed great respect towards her and where it was not at all a rushed process, but a very slow paced tranquil one. The atmosphere within that clinic being notably calm and peaceful, where it made death appear to be of secondary nature to that of the transition from this life to the next one.
If death is inevitable then ending one's life to avoid a slow painful death is simply assisting the natural process of events.
Although, if it was made legal then it could be open to abuse, where for example a patient may be given the choice of whether to live and/or to die as a natural course of medical procedure. What would unsuspecting patients feel like when suddenly having such a question posed to themselves when they were least expecting it?
Also looking into the distant future, where this might seem extreme to us now, we might even find that the next step along the way would be to terminate people's lives as soon as they have reached a certain age. It might first be a termination age of 90, which is then reduced to 85 and so on, where eventually we could end up with only a youthful populations here on earth.
The other alternative to the age factor could be the type of disease factor taking into consideration the various stages of those diseases and terminate people's lives as soon as they had reached a certain stage of those diseases etc.
Everyone has different degrees of inner strength and different degrees of pain threshold, where someone might have a low threshold of pain in contrast to someone else who might have a high threshold of pain, so what might be unbearable to one person might be bearable to another. One person's body may be also more resilient to a particular disease than someone else's.
Therefore, there can be no hard and fast rule when considering termination of life.
We have on one hand our earthly human bodies, which act as temporary vessels for our souls, chi, and whatever are our beliefs, but on the other hand we have our religious/spiritual selves, our religious/spiritual lives, where each one of us deserves individual respect regardless of our age and/or any disease we may have, so the right to end ones life should be one of personal choice, and where we should not have that choice either posed to us and/or made for us.
