Science doesn't care what you think, or care for your preferences (nor mine) [edit: meaning that our personnel views on things without applying the scientific method are typically wrong, just because I say not having gravity would be favorable because humans would live longer does not make it so, there is a probability that my belief / preference for a lack of gravity , and the removal of gravity would make humans live longer (although that probability is unknown until a series of events is observed), but until determined to be a fact (fact defined: as a close agreement of a series of observations of the same phenomenon) the belief should not be taken as true.
What do you even mean when you say life "matters". Are we talking about the duration of life? Are we talking about the quality of life? How do you want to measure it?
Hypothesis:
If we do not euthanize the elderly (define elderly in terms of age) or sick (define sick in precise terms for the experiment) ____ will occur
What if an "atheist society [i guess your defining it as one that performs eugenics, without a belief in god?]" was able to improve the duration of life via euthanasia and eugenic practices? What if the number of non lethal diseases experienced in a lifetime was reduced from the use of eugenics practices?
What is to stop a "religious [one that holds the belief there is a higher power]" society from practicing eugenics (to make man more into the image of "god" [god being defined as whatever traits the particular religious organization desires])? Or from "euthanizing" non believers (lets relief them from the pain and suffering of sin!)?
Before we can even begin to verify what will happen we need a better definition of religious society, and atheistic society, and from there we would need to study it empirically.
And to be honest I don't see how a belief or lack there of in god is relevant to the use of eugenics, or euthanasia on society.
Your also not defining a religious society at all. What is it a society in which X number of sample participants choose between claiming to be religious, and claiming not to be religious? You would also have to clarify what you meant by religious because many practices/beliefs are quite different, but associated with the term.
Just some things you should take into account.
Cheers,
Dynamic.