Leave someone else's dna at a crime you've framed someone, how did you get that dna, well you bought it from some unscrupulous lab. Or how about insurance companies start purchasing this data, oh but thats illegal so lets just say we don't but actually really do and totally pervert the idea of insurance. Golden state killer might stick out in your mind as a horrible evil but its nothing against the trillions of cuts by making this data widely available for purchase.