How can they be guilty? Does it mean they are only divorcing now because Malini found KC? Else Adi would have stayed with her? If that premise itself is wrong as clearly stated by Adi - when he said their relationship would anyway have broken - then why would there be guilt? Adi wanted to stay in a badsoorat relationship?
Adi would have divorced Malini whether KC was involved or not, there is not argument on that. The guilt will always be over involving her in this mess. Her having KC was making him feel a bit better that she will have a support system to be with her during this entire process, so finding out that she lied just to "get him to move on with Imlie" will make him feel bad, ke usne yeh sab mere khatir kiya types.
Adi is a straight guy, so while the situations have been instrumental in creating the mess, Adi's choices have also contributed equally. Either he should have let go of Imlie completely, else told Malini everything if he wanted a completely clear conscience. But circumstances, his choices and situations all got together to bring them to this point. Adi shayad Malini ke saath reh bhi leta had Imlie not come in his life at all. He realized a lot of the incompatibilities in his relationship with Malini because he had a comparison point. I am not saying he would be happy with Malini, definitely not as they are not suitable together in anyway. But because he had no reference point, they would have carried on with the relationship in a similar way as Malini's parents did. Adini's relationship always had a parallel to Dev & Anu's relationship, so I do not know if Adi would have at any point realized that him and Malini do not work. They had been together for 7 years and thought that their choices matched and could talk for hours, but both of them hardly had any heart to heart conversations about what they truly feel. So I really do feel they would have continued that delusions if Imlie had not come in their lives.
Imlie was able to show Adi the mirror to his own hidden personality traits that he did not even know himself. So it was very difficult for him to remain the person that he was with Malini earlier. That badsoorat relationship became clearer to Adi, because of his feelings towards Imlie. Imlie did not come between them, she just showed Adi a side of him that he was not even aware of and could not ignore anymore, just to keep Malini happy.