Fergus
Death Eaters have no established history of targeting purebloods who don't speak out against them. If she was killed by one of them, she probably got caught in the crossfire trying to get an injured person to safety. It's entirely possible that she was accidentally killed by a member of the Order, too, depending on how close to the fighting she was at the time.
Regardless, we don't know, and putting someone on the defensive is not a good tactic for getting information out of them.