It is just to create suspense and drama because if he was really that evil he would have not lead gunjun aman marriage and there was even no need to marry preeto he would have simply told her truth
I think it's well planned by both of them or only Raj at spur of moment so that he could become Shah and get his badla from mohini
I think after marriage or even before marriage they both have planned which will be revealed later
If he really turn out evil then there no point of story it would be some saas bahu story which saris is not
Anyway let's see
If he will be really evil then I will never want rajpreet i will want new male lead because in my eyes no girl deserve such a guy