The way NK loved Khushi unconditionally, understood her discomfort and tried to cheer her up when she was feeling down shows how a perfect human being he was. Khushi was blaming herself for his death, even though she wasn't at fault.
Arnav's redemption was so heart breaking. All his hopes have been shattered just because of a misunderstanding. Not only he lost his cousin brother but also he lost the Khushi he knew. It grieved him to see her in such pain. He also held himself responsible for NK's death just like Khushi did.
At the end, there is always a ray of hope for them even in the darkest of times. They can heal each other's pain.