I find it hard to believe Durgesh's theory. Of course, their parents loved them. Babuji loved them too but I don't think that after they did to him and Maaji, he would really want them not to be punished.
Durgesh wants to change those devils. But I don't think such people can change. I am not saying that people can't change at all. Of course they can… but that's only people.. Humans. We all make mistakes and can/should be forgiven but if a person can stoop too low as to kill their own father and torture their mother, then I don't think the person is human anymore. They have crossed all the limits of humanity.
Of course, humans can change and regret/repent for their mistakes but people who aren't even humans anymore can never change.
And as it is mistakes can be forgiven but not sins. I totally agree with Kshitij on this thought, that she cannot change them.
People who are forced to become evil due to circumstances or force can change because that's not what they intended to do in the first place. But people who are bad nature wise and do everything bad thing consciously can never change because that's their nature and no one's nature can be changed.
Of course, it's a show so probably after awhile due to Durgesh's techniques, they might change but I don't think in real life that can happen at all..