Does 1 good deed cancel off 1 wrong?

I was on the MRT last weekend when I saw a Mainland Chinese mother with her daughter board the train at Bugis station. In the girl's hands was a cup of ice-cream. They managed to get only one seat. So the mother seated her daughter on her lap. The daugther then stated spooning little portions of the ice-cream into her mouth.

Further into the journey, the passenger beside them vacated. The daughter was then placed on the empty seat. Then a pair of filipino passengers boarded. But there was only one vacant seat. One of them took the seat anyway. Then the girl gave away her seat to the other filipino and went back to sit on her mother's lap. Though one of the reasons was that the seat was actually still warm from the previous passenger. She was still nibbling on her ice-cream.

I had the intention to show the mother and daughter the "no food allowed" sign, but they left before I could say anything.

Anyway, the incident got me thinking. Does 1 good deed (giving up her seat to others) cancel off 1 wrong (eating on the train)? Also, the girl never dripped ice-cream in the train.