This slap came more on what Gulaal sees that he is crossing his limits and also what state she is in at the moment and how broken she is as well, her emotions have been opened up whilst before she had control over them and also backed up her slap and how she let her some of her anger out in him - though what she said wasnt wrong, back to the scene, i watched the slap scene again coz if you watched what she says after the slap it makes sense to some extent, that this family is going through so much at the moment and had been in the past, and everyone is trying to keep everyone together, give that certain respect and so on esp as she mentions the elders of the family, whilst Kesar seems to be making everything worse and causing friction and so on - see that how it looks to ppl outside in how Kesar is behaving and so on
So no wonder how it looks to other family members and Gulaal, she has seen first hand how angry and so on Kesar actsSee as viewers, we totally get what Kesar is saying and doing and feel for him, and at the same as viewers we can see that he was speaking so out of turn and in the way he was speaking that a slap was due, the fact that it came from Gulaal gave it maximum impact and will shape up Kesar's mind and hurt from now on as well - only Gulaal could of got the reaction we got from Kesar at the point, the utter surprise and hurt and tears and silence from Kesar and to back away from her and leaving and then seeing Gulaal's reaction after he left as wellAnd D at the end says, this anger of Kesar seems like we make him even more distance from them allYou only realise the impact when afterwards in what you have done and this happened to Gulaal, she realised what she had done, and maybe she shouldnt have slapped him, how shaken up she felt and worried and hurt as well