The argument for assisted dying is most strongly felt by people who have slow acting terminal diseases. The arguments against are the "slippery slope" argument plus a belief that it is usurping God's will. But if you are an atheist, one argument is gone immediately. Prolific and very popular British author and humanist Sir Terry Pratchett, (born 1948), publicly announced in December 2007 that he was suffering from early-onset Alzheimer's disease.
On February 1, 2010 he gave the Richard Dimbleby Lecture - the title: Terry Pratchett: Shaking Hands with Death. It was ably delivered by his friend, Tony Robinson. He makes the case that modern society, confronted with an increasingly older population, many of whom will suffer from incurable illnesses, will need to redefine how it deals with death. The Richard Dimbleby Lecture is delivered each year by an influential business or political figure but this is the first time by a novelist.
See the last and best part here. (Tony Robinson is delivering the Lecture with Terry Pratchett watching)