I seriously think that how you behave will really impact on your child next time, but I don't understand why some people can teach their children to be revengeful next time when they grow old~

Totally agree, whatever we say and do affects the development of the child however, let's not judge those who teaches their child to be revengeful. It could be because they have been hurt too many times. Anger is one of the easiest emotion to portray.