Kunal cheated on Mauli but he was also loyal to her for 7 years, they were together for 7 years... In all those 7 years he didn't sleep with every woman he come across, he cheated on Mauli but he is not a habitual cheater...
Pre leap -
1 Kunal is a man who wouldn't take money over his patients, he left his job because his boss asked him to write more tests to every patient that comes to him
2 Kunal gave Mauli her space and respected her profession, he did have problems with Mauli not spending as much time with him but he didn't really fight over it, he was very understanding...
3 Kunal wasn't an insecure husband, Mauli and Kunal share same profession and Mauli was obviously more successful, if you could look up the record you'd see many husbands are insecure when their wives earn more than them.. But Kunal celebrated Mauli's success
4 Mauli helped her friend best friend Nandini... How would she do that if her husband didn't allow her to bring her best friend in their home? Kunal supported her then too
5 Kunal helped a stranger Nandini... Remember that scene when they were in goa and Rajdeep threw Nandini out of the room, Kunal passed that Corridor where Nandini tried to hide herself in shame, he lowered his gaze and moved past her to make sure she doesn't feel ashamed, later when he saw her come under a car, he saved her...
6 Mauli got Nandini home, but who was the one given responsibility of Nandini? Kunal, right? It was always Kunal who had to take Nandini here or do something for her, when Nandini had to go to bank, Kunal helped her gain her confidence, he didn't have any ulterior motives at that time, he was simply helping his wife's best friend, do watch the scene where he helps Nandini open a bank account
He was absolutely insensitive and inhumane during EMA but wasn't he the one who told Mauli that - "I am sorry Mauli, mai tumhara gunehgar hu, jo saja dogi wo manjur hai?"
Later on he was very insulting and doubted her pregnancy, blamed her for Nandini's absence
Post leap -
1 He is an amazing father to his daughter, it's a responsibility and everyone has to fulfill, agreed... But the same responsibility is was on Mauli, but did you see Mauli actually helping Mishti when she was psychologically troubled over losing her Popsie? Didn't Kunal understand Mishti's behavior and help her? Compare Mishti's behavior with everyone before Kunal helped and after it, how many time Mishti tell Mauli that she hates her?
2 He's still a good doctor
3 When Mauli planned his birthday in camp, she berated him lrt, he apologized to her once again, not for marrying Nandini, how can he apologize for marrying Nandini, if he believes that he loves her? But he has apologized many times for hurting Mauli...
4 When he realized that Mauli hasn't moved on, he came up with a plan to make sure he looks bad in her eyes and simultaneously helped Ishaan, he was the one who called Sandhya and Sandhya agreed to take Mauli as her daughter in law
Everyone has flaws, not everyone is mahan, in real world people are selfish, selfishness is a part of animal behavior...
His Karma -
He did all the bad things to be with Nandini, when a person stoops so low losing everything he held close to have one person but he lost her as well, they have a daughter together but he lost Nandini to cancer, he had to take care of his daughter knowing that any day the love of his life will die. Cancer patients are bedridden in last stage, there is chemotherapy and all, this is not a one or two month's procedure, this takes years, Pari said that she hasn't seen her mom, so Nandini must have already had cancer for her to die so early...
We all call him cheater and all but after years of his wife's death he is faithful to her, isn't he?
He is not a habitual cheater...
Note - This is in the case where Kunal loves Nandini only not Mauli...