If your family member is a mass murderer, I really don't care if investigators use information related to you to catch them. Even if that data is your DNA, and it was shared contrary to your expectations.
Nobody wants to see their loved ones go to prison, but the Golden State Killer was an absolute monster, and it's an demonstrably good thing he was caught and faced justice.
I understand that people feel that using DNA information from genealogy services to investigate crimes might feel like a slippery slope towards egregious abuse (and maybe that's true), but the Golden State Killer is definitely not an example of such abuse.