We don't know anywhere near enough about the Ghostbloods(or really Jasnah) to be able to guess why the Ghostblood's might want to kill her. They may believe that killing her is the best way to get a greater fabrial, They may know she is researching the void bringers, they may believe that they need to kill and surgebinders to prevent desolations. Any one of those three i feel is equally plausible and im sure there are many more options i haven't considered