The ancestry databases (at least, the ones I'm aware of) contain SNPs and not full genomic sequences. Even if "targeted assassination virus" was a real thing, it would almost certainly need more data.
Seems like a lot of work when a bullet or dioxin is much cheaper. In any case, if they really wanted to off someone with a targeted genetic attack, surely they could just swipe their DNA from a door handle, or a piece of mail, or a piece of trash carelessly thrown out, or from their sewage line, or a million different ways.