Kreznik was in a state of total disbelief the entire time Lavender explained herself. She had suspected him for so long and didn't care!
Finding out what kind of person the real Nick had been put that in perspective though. A lot of things he had thought didn't add up during his time here finally made sense. Nick hadn't been a good husband or father AT ALL.
His heart ached. Poor Lavender. She was so sweet and snuggly; it must have been really hard for her dealing with someone cold and uncaring. She saw her chance at being treated well and took it. How could he possibly think she was terrible for that? He had done so much worse.
She loved him. She admitted that she had loved HIM not Nick almost the entire time she had known the truth. She said she needed him.