[Deep2]

Your case is flawed. You have argued she should not be blamed for what she does, as others do it all the time. Yes other people do back stab, etc. But from MY discussions with people, it is not the fact she is back stabbing, but the WAY in which she does it. She is truly odious and snobbish in her approach, dismissing other housemates for no reason but because she feels superior.
One cannot merely look at the actus reus (the back stabbing), but also the mens rea (the thought behind the action). In Grace's case, the latter is merely to put others down because she actually thinks she is some princess, as girls have said to me, she is just like bullies they encountered at school.
Others who also back stab have a varying mens rea from Leah who is clearly one of the most insecure people there, to Lisa, who is plum ignorant.