Discovering that your lover is actually the enemy whom you swore revenge against is not a simple thing. It wouldn't have felt right at that point in their relationship to just break up and disappear, but also I would have been really mad if Gu Mang forgave him because "he didn't know" or something. No! Torment the man! Make Lu Chengzhou panic, give him a heart attack, do something that he will later dream about and wake up in a cold sweat!!! It's because Gu Mang trusted and loved him that learning he was the one behind her pain hurt her so much, and it's because Lu Chengzhou loved and cared for her that he inflicted the same pain on himself in order to make it up to her. They let their actions speak louder than their words, and that's why it felt right for them to overcome this problem and stay together.

Also, love Gu Si for spearheading the whole "make my brother-in-law suffer" plan. I don't doubt that he would have taken Lu Chengzhou's head if he was able to, but it's still funnier that he just decided to raise as much hell as he could and nobody suspected him because he's like, ten years old.