If you can infer things about living relatives from a DNA sample (preexisting conditions, for example), then you should need their consent to release that sample. It’s not only your information.

This is a genuinely interesting ethical question. While our phenotypes (bodies) are separate, our genotypes are very much a shared resource (at least for read access); an extra special case are monozygotic twins, where if you obtain sample from one of them, you just mapped both.

Fortunately we don't see applications like "personalized poisons" yet, but it is likely inevitable.

If, say, an insurance company denies you some policy because of what they learnt from your relative's DNA, you suffered a concrete harm from that sampling decision.

Ironically, I think it’s this kind of attitude that creates an environment where the parent is rightfully upset.

We are not isolated units. Almost all our choices have impact on others. Lack of a shared culture creates societies where people are rightfully scared what the next isolated unit will do with their sensitive data.

It’s possible to live in a high trust society.

because if they murder someone, they'll be easily traceable if they leave genetic data behind

Because I don't like what her DNA can say about me or my kids. This woman and me share enough DNA for this to be potentially pretty rough.

Secondly I don't trust anyone with that information because even though I trust how it might be used today, I don't know how it might be used in ten years.