I mean she was married to a man, who did not love her.He was always in love with someone else and married her for duty and kept seeing his first love.I think it would make anyone bitter.
I would be a bitter person if my husband did that.Not only that he also has a child with the other woman.
It was amazing to me when Sums said she would not let Disha do to Pori what happened to her.In her case it was the other way round.For whatever reasons Dhruv married Gargi, she should have gotten out of his life.
I am not saying it justifies her horrible behaviour but I can understand why she is so bitter and mean.