Because he didn't want to kill him and Frank was ordered to kill only the parents not their children
Makes sense. I thought when he was looking for the kids, he'll also kill them. I guess he only wants to see what the children's lives turned out, him being gifted but being used. Thanks
There was a car chase scene where Gye Do chased after Frank after his father's death. Frank realized that he was Bongpyeong's son after beating him. Then they didn't show what happened next.