@sidra - I think what changed in him was his ability to empathise with her. Previously when he first joined the kabiley as Shatru, he could only see her as the enemy and somebody he has to manipulate in order to complete his duty. Perhaps in between he felt something, but he kept it under his feelings of revenge.
I'm guessing all this would have gone on as such if it weren't for the drive that they had. Imagine if you really really hated a person, but for some reason one entire night the two of you can get along and actually enjoy each others' company. For once you're more like 2 ordinary people having a night out instead of being enemies.So basically, at least for that night, Abhay's freshest memory of Phulwa would not be of the times they kept arguing but of the simple night they had. Moving along, now if that person were to suddenly get badly hurt, your first reaction would be to help him. That's the same amount of concern Abhay was showing not for his enemy but for Phulwa.At the hospital when he was looking at the bangle, his overriding concern for her gave him the ability to be able to see the whole situation from her point of view and he realised how he had played with her feelings the whole time and the humiliation he made her suffer. His guilt would increase ten-fold due to the fact that she was actually dying at that point in time, because you'd feel really guilty if you couldn't apologise for your actions.I think what hit him the most was the fact that despite it all, she was still calling out his name, concerned for him. And that was what pushed his guilt even further, making him sit with her and hold onto her hand. And also the reason why he can't forget how bad he was to her.So I'm guessing whatever he's doing now is a form of repentance towards her, combined with some half-baked feelings he probably has for her. But I guess whenever he's around her he tends to forget the guilt part and gets mesmerised by the half-baked feelings😆That was kinda just my take on it, makes some sort of sense to me but I'm sure there're plenty of other reasonable reasons out there to:)