I agree that Kiran is no saint but if you look at what's going on from a third person's perspective then Abhi-Simran are much more at fault than Kiran.
Look at it this way, Kiran entered Abhi's life when he was married to Simran, she enticed him and started an affair with him. Now here Abhi is more to be blamed than Kiran. He cheated on Simran and married Kiran. Simran like a stupid fool gave him full permission for the wedding. As long as Kiran stayed 'married' to Abhi, she was faithful to him but he kept thinking of Simran all the time. Then he ditched Kiran as well becoz he thought she was too ambitious and too modern-thinking. It's no surprise that Kiran, like Neha was insecure and jealous of Simran. So when chance threw Aastha in her way, no doubt she's going to take full advantage of the situation to get back at Abhi and Simran.
Abhi and Simran meanwhile failed to take control of the situation. Abhi failed to reveal the true nature of Kiran to Aastha, Simran failed to inspire any kind of trust or affection in Aastha. As a result of their poor parenting techniques, Aastha moved closer to Kiran. I am not defending Kiran here but if Abhi-Simran were aware of what she's leading Aastha into, they should have taken some drastic steps. They seem more concerned about the award ceremony and their oversees trip than finding out abt what's happening with Aastha. They just can't let her go, thinking that she'll change her mind one day. If they were really caring, good parents they would have gotten Aastha away from Kiran no matter what. But they don't even have the basic desire for doing so and are running away from the problem..so typical of them 😡
I feel really sorry for Aastha and angry as I am with Kiran for her deceitful ways, I am more angry with Abhi and Simran for all this mess they have created 😡