jumping in here + follow up to my earlier comment -
I think his obsession, even if there's a secondary reason as Sunny was hinting at, is now so much that he's inching closer to the mindset of "if I can't have Pallavi, no one can." Or he's willing to punish her to get his way. If that means she gets hurt, then she does. It doesn't matter to him.
Just watched the burning saree scene -
Yeah he was shocked at first, and maybe fascination is a stretch, but he still seemed... mesmerized by the flames? Even when he came down the stairs, he still just looked at the fire as it burned. He was barely looking at Pallavi - he was looking more at the saree burning.
My only other thought is that it triggered something in him. Maybe a memory of his accident post leaving the hospital?