I have discussed this so many times with my friends.. the conclusion was shivay always felt that he knew Anika. however she said yes to marry Daksh when he said she wouldn't. She said it coz she wanted to prove to Shivay she isn't affected by him. But to him that meant he hardly knew her as much as he thought he did. Then the misunderstanding created by Daksh, it was easier for him to believe that anika could sleep with Daksh for money coz he thinks with his mind and not his heart. He was always like that.
Now coming to the marriage, 2 brides running away and he was desperate... and desperate times call for desperate measures. The only option to save his reputation, family name and stock prices from falling was to marry the only girl available at that time. And so he did, though she never fit in the criteria for being the wife of SSO at that time. But he didn't have any option. He this became the beast he did coz he had to make her agree and she wouldn't agree without being blackmailed hard into it.PS: I am not justifying what he did. I condemn him too and am no where supporting him.