What was the reason for him to turn evil ? Wasn't it Rio ? Rio created misunderstanding between him and Vaishnavi and now he will marry her. So, he became evil. I know it's wrong and I don't support it but the real reason for Parth to turn evil is Rio.
No they have completely changed the character of Parth, it was a big revelation
Parth and Rio were working in cohoots. Parth wanted money and so he played a bechara kind along with Rio. So that now Karan gave him the POA of his business, Parth is willing to sell Ba's property
Vaishnavi and Parth are now married and he treats her badly
Thing is Rio is doing that since he thinks his dad was wronged, but Parth is real evil, he is doing everything just for money
It is highly probable that this time it will be Parth who does the MR