This is the end of the anger road. The behavior has become an end in itself. Your child's hurting someone because he was angry with them was a serious problem; but hurting them just to see them suffer is very disturbed behavior.
Although a little teasing and tormenting among children usually are only good fun, too much is a serious problem. It works like this. When children are about five or six, they begin to see things as others see them. It is as if they momentarily become the other person and feel what they feel. This is called empathy. By grade school, they do this fairly automatically most of the time. One value of empathy is knowing when other people are hurting physically or emotionally. If they are, your child feels badly and wants to help. This is how he tells when enough teasing and tormenting are enough. When it stops being fun and starts hurting, enough is enough. Knowing just when to stop is a problem; but most children know about where the line is and respect it.
Children who are intentionally cruel and get pleasure from the behavior are across the line. They want to see the other person suffer. Empathy or the pain of the other person does not stop the behavior. Usually, children showing this sign have themselves been victims of abusive behavior and have well-learned what they were taught.
Be sure no one in your home torments your child physically or emotionally. Further, keep teasing to a minimum and assure no one tries to retaliate against your child when he does something for which others may want retribution. Neither you nor anyone else in your family should try to teach him a lesson by hurting him. He needs gentleness, firm guidance, and a predictable family. Your primary approach needs to be talking with him about his behavior, the feelings and pain of others, and about his angry emotions.
A related sign involves sexually aggressive behavior. This is usually perpetrated by boys on girls but may involve children of either sex with children of either sex. Typically, the behavior involves an older or bigger youngster forcing sexual involvement with a younger or smaller child. The key is the element of coercion and the inability of the victim to stop or prevent the behavior. If this type of behavior comes up with your child, take it very seriously. It is not a "children will be children" kind of thing and often indicates a serious behavior disorder requiring immediate, specialized attention. At a minimum, discuss the behavior with a specialist in the area of children's sexually disordered behavior, keeping in mind most "therapists" are not experts in this area. Similarly, the victim of sexually aggressive behavior needs specialized help too. Most therapists who work successfully with children have expertise with victims.