The message the movie is trying to say is commendable. Don't physically abuse (in this case a slap) anyone especially your partner under no circumstance.
People know this already and if they think that this movie gave them something to learn then the mindset of people is bad. And the movie is not so good in my opinion, it's just a film that caters to feminists and extreme leftists. Taking a divorce for a slap on her cheek is laughable. Even in the most culturally advanced and developed countries would laugh at this movie. The acting is very good especially Tapsee Pannu.
Yes it's wrong to slap anyone and the husband haven't even said a sorry for his action afterwards, I think no one in this world would not say a sorry after that kind of a mistake. This movie is just saying 'Don't slap your partner, if you do she will suffer from depression the rest of her life or she will divorce you'. Just switch the genders and make the same movie.