It is not written that all evil has to rule and that it is handed down to your offspring. In some cases this is true, but in a lot of cases, the children of these evil people don't want anything to do with them at all. They try their best to be completely different than their parents.
In the Harry Potter series, it has always been a fight between good and evil with Harry and Malfoy. In the beginning, I had the feeling it was Malfoy who was so jealous of Harry and his abilities that drove him to evil. Maybe evil was always in his soul and this was just who he was.
As a son of the evil, it is good that the storytellers didn't carry this on to the next generation and have decided to show people that the product of good can be born from the product of evil. I think this turn of event is good and will really add to and enhance the story. It also adds a nice new interesting twist to the plot.