Peter Singer frankly distinguishes Christian ethics from secular ethics:
Any discussion of the ethics of voluntary euthanasia must begin by considering whether it can ever be right to kill an innocent human being. The view that this can never be right gains its strongest support from religious doctrines that claim that only humans are made in the image of God, or that only humans have an immortal soul, or that God gave us dominion over the animals-meaning that we can kill them if we wish-but reserved to himself dominion over human beings. Reject these ideas, and it is difficult to think of any morally relevant properties that separate human beings with severe brain damage or other major intellectual disabilities from nonhuman animals at a similar mental level. For why should the fact that a being is a member of our species make it worse to kill that being than it is to kill a member of another species, if the two individuals have similar intellectual abilities or if the nonhuman has superior intellectual abilities?