Ordinarily, I don't get into news stories and social media memes in which everyone piles tons of fairly obvious scorn upon specific individuals who are especially ignorant or depraved. I'm more of a systems advocacy kind of person, and hating on even very hateful individuals, I think, tends to divert energy away from dealing with institutional problems.
That said, I do think that this is an exceptional case. The letter clearly and specifically progresses from the writer's personal annoyance, directly to some very disturbing life and death ideologies. It's considered bad form to invoke Hitler in a debate, but in this case, the Nazi comparison is quite apt.
I'm not genuinely afraid that this signals some fundamental societal shift in that direction. But it does put the lie to a pretty common fallacy among some of us in the disability community, myself included … that ableism is mostly benign ignorance, and that people don't really "hate" us. "Pissed off mother's" letter is a useful reminder that not too far underneath our veneer of enlightened progressivism, there's still a lizard brain that is disgusted by and afraid of "others". And people who look funny, move funny, act funny, and sound funny are definitely "others."
Don't get me wrong, I'm grateful for the veneer, and I think lots of people don't react negatively at all to people with disabilities, even down in their lizard brains. But we'd be fools to think that a mere 50 or so years of relatively enlightened behavior and policy have entirely undone the impulses that led to forced sterilization, warehousing, freak shows, and the Nazi T4 program.