What Viaan did cannot be justified.But he has truly repented for his actions and redeemed himself.He has asked for forgiveness countless number of times.I don't think he can do anything beyond that.Katha fell in love with the changed Viaan and made a lot of tall claims and promises to him.But she broke all of them for whatever reasons.The biggest problem about their relationship is the fact that there is always huge imbalance.Initially it was Viaan who misused his power and took advantage of Katha's helplessness.Now it is always Viaan trying to work on their relationship.A relationship of equals is not always possible but such a skewed relationship is not good either.
@True, it is more like she doesn't want to talk about that night. It still gives her chilling affect.
However she made a very big lecture to Maya, what happen now. Why she can't explain Viaan is changed when they have witnessed risking his life in saving Aarav and accepting Katha even if she is not able to become pregnant.