Like in the typical love triangles...especially when the amn is amrried to both the ladies, one has to die and they cannot show Nandini dead because of all the chemistry between Nandini and Karan. Now showing Tanya dead for no reason is also bad as she sacrifised her whole life for Karan and Bhoomi. THe reason she changed so rude and selfish is either the director of this serial istryint to show he bad, in other words preparing the audience so taht we will hate her,
Also, like the typical movies, she is suffering from some severe health issue and in order not to hurt Karan or anuone else, she is trying to be bad to them so that Karan will hate her.