The thing is, none of this is even logical or rational. They are just trying to find excuses to push evil behavior into society using terminology that makes it sound "informed", but the fact is that they don't get to make a new definition of what constitutes an "actual person" or the "moral right to live". I'm sure they would be much more careful when dealing with animals and their rights.
Ironically enough, Peter Singer does promote vegetarianism and animal rights.
At the same time, the eugenicists often make the claim that fetuses and infants have the same level of perception and sentience, or less, as many animals do, and since we kill relatively intelligent animals like pigs and cattle with no qualms, that we should be able to "reduce suffering" by killing disabled infants or fetuses.
They see nothing inherently different about the fetus or infant being a human being. They think of people as just another animal on the same moral level.
This is why PETA posted up pictures of chickens in a slaughterhouse and called it another Holocaust. To them, it's exactly the same and just as morally repugnant.
Singer and Dawkins appeared in a series of youtube videos where they discussed both vegetarianism and cannibalism. They said cannibalism is just the same morally as eating other kinds of meat.
Even though I accept that evolution was a method that G-d used to create, I don't believe that animals and people are morally or spiritually equivalent. Unfortunately evolution as a concept has been used for evil purposes such as promoting eugenics or trying to morally equate humans and animals, and I think it would probably be better if most people didn't accept it or believe in it simply for the fact that they wouldn't be as subject to these types of arguments, and traditional creationism automatically puts people as separate and apart from the rest of life.
This is also to a large part a result of government-provided health care. Society will look at parents with disabled children with hate and resentment wondering why they didn't "do something about the problem" before it cost the society money. Of course they don't say things like that when it comes to third worlders who leach off of Western systems but that would be much more politically incorrect than advocating infanticide, sadly enough.