Hard question. Really, really hard with no easy answers.
I've had one parent die a horrible death (ALS) and I have one that I am positive is on his way out. He has a living will and a DNR that will allow him to die, mainly through withholding medical treatment. I think I can do that as I will be the one making the decision and I know it's his desire, but at the same time, I am so afraid I won't have the guts. I love my dad and don't want him to die, but I agree with him that there is a point when life is no longer worth living. He's at that point. He has no joy, no happiness, and is so damn lonely that I believe it is literally killing him. He does not want to be in this material world any more. There is a part of me that hopes he passes because his quality of life is so bad, but then there obviously is another part that does not want him to go. It's excruciating to watch him decline.