Abhay is a multi layered character. On one hand he is the best son, on the other a ruthless sort of police officer. He is fanatical about what he does and doesn't care which way he has to take in order to achieve the end result. Like Dadi said he is very passionate about doing things whether it's doing good for someone good or punishing someone.
And that's what happened here. When he learned about Phulwa standing up against his father he went to all extremes to teach her a lesson.
When it comes to his loved ones he is ready to do anything to protect him. And he believes that he is doing right. If you see from his point in a way he is right because he is just protecting his family but sometimes he can get harsh to get his way. And its his nature too. I mean he is shown like an aggressive man. Even in his scenes with Aarti whom he liked once, he was most of the times...stern. So him behaving badly with Phulwa can be attributed to his nature as well as the fact that he still is unaware of the truth.
And no, im not trying to justify his act but trying to figure out where he is coming from.
And in doing so im hoping that like he went to extreme to punish Phulwa, once when he will learn the truth I hope he will repent with the same severity and try to do the right thing. :)