It's not just that -- the National Socialist German Worker's Party thought that they *were* promoting human life and prosperity by removing those that they considered "unfit" -- not just for religious reasons, but the mentally challenged, disabled, *especially* those with birth defects, etc. In their minds, they *were* promoting human life and prosperity by considering "quality of life", and believing that everyone would be more prosperous if there were less people (they looked at people as takers more than providers).
Today, in the United States, the Socialist Worker's Party is advocating a health care system that considers the "quality of life" of people before making a decision as to whether to keep them around or terminate them (voluntarily, of course). A case in point is the woman in Oregon who wrote to the administrator of that state's State-Run Healthcare system, pleading for them to approve cancer medicine that would save her life. She got a letter back saying that it was too much of a risk, and reminding her that Assisted Suicide *was* covered by her plan for a co-pay of only $200. How is that moral?