If someone is suffering from an incurable disease and just want to be let out of their misery, just give it to them as painless as possible.
For example, if I found out I had Fatal Insomnia, I'd probably try and find the fastest and easiest way to end my self, before I suffer from that. If you don't know what that is, you can basically never ever ever ever fall asleep ever again. You'll basically go crazy and die within a month or so, it's scary as fuck. Thankfully, it's very very very rare. And you only inherit it. Also, there is NO cure. You're FUCKED.
https://en.wikipedia.org/wiki/Fatal_insomnia
As for Eugenics, if I found out I had a chance of having Fatal Insomnia, I would never have a kid. I'd get my self chemically castrated. I would never want to pass on that suffering to anyone. I don't think Eugenics is that bad of a thing, just the US and Germany took it in a racist approach. If we used it as a way to clear awful disease from our gene pool, it would be a good thing. But if someone does have a disease in their gene, you'd have to find a way to get them to agree to chemical castration, and not force it on them, lol. Perhaps offer some money? I dunno, lol.
But hey China is gene editing babies, it's pretty much Eugenics but much faster, and a lot less racist. Perhaps in 10 to 20 years they'll perfect it. If they did perfect it, and if I found out I was a gene baby, never had to worry about cancel, or old people diseases, I'd be thrilled!