This isn't about revenge. It is about a kid being kicked silly daily and finally having the means to defend himself. The mother is telling her son to beat the snot out of this other kid if he is attacked again and it is very likely he will be. Are you advocating this kid simply keep taking beatings from another when he has the means to defend himself?
When people attack others, they deserve what they get, even if it is death or maiming. Don't want to die or be maimed? Don't attack someone without just cause(I.E. self defense). I hope this kid beats the sh** out of the bully if he is attacked again. He may actually save some other kids life, because quite frequently bullies grow up to be violent criminals if they are never brought to taks.
When a student tells the teacher that he wants to learn a martial art so he can beat up a specific person; that's pretty much revenge. If the student had an ounce of common sense he would say "So I am not beaten by bully's". One shows intent to harm, the other states a desire to protect oneself.
If you worked in a Gun Shop, and a person said "I want to buy a gun because I want to blow a hole in Billy Bob Jones" - would you sell him a gun? Probably not, because you would be aiding in the commission of a pre-meditated felony act, and you would risk sharing in the punishment phase.
Same scenario, the guy says "I want to buy a gun because I want to blow a hole in any burglar that breaks into my home". That's the difference.