Autism for instance is not a death sentence, but it could certainly be considered a fate worse than death. I wouldn't blame a parent who wants to put their autistic child to sleep. We're talking 80 years of being a helpless vegetable who bangs his head against the wall and can't speak, only moans when he wants something. Or 80 years of being a socially inept, ostracized leper who has no friends and no hope of ever being "normal." Silicon Valley notwithstanding, ForeverAlone and unloved isn't exactly a picnic. And there's not even any treatment for autism that can "fix" the person to live a normal life and be a normal person, like there is medication for bipolar disorder or depression that can "fix" the person's mood. There's always some degree of spergishness about them that can't really be suppressed.
I mean really, would anyone want that kind of a curse for their kids? A life where, depending on how the coin is flipped, you're either Hulk 24/7 aka I Have No Mouth and I Must Scream, or you're Sheldon Cooper and everyone hates you, and you know they do but you're so inept you don't get why (and therefore can't