The Death Eaters have shown me that it is frightfully easy to kidnap them even behind the wards of Riddle Manor and after many many days of checking after taking care of Bella I've found that despite knowing she's missing none of them, not even the Dark Lord , suspect that she is either missing not of her own free will or dead. Speaking of I buried Bella under a willow tree near Hogwarts in the Forbidden Forest. Looking through her memories It was the only place she ever felt free. There's  a marker under there with the name Bella and the inscription "Death will set me free'. I asked (read: begged) the centaurs to maintain it and encountered very little resistance. Being more spiritually inclined than your average witch or wizard has it's benefits.