Odd girl out

Usually pretty girls are the popular ones.I was friends with one of the popular girls back then and you know they are selfish,conceited, and come with attitude.Popular girls believe that they should bring other people down to make themselves feel like they are on top of the world.There was a time where one of my friends,the popular one,we were just walking around the school when she saw a girl that was sitting by herself,and she decided to go up to her and started to make fun of her asking her why she is ugly,why she is a loner,why she is talking to someone else's boyfriend,and etc.At that point i felt bad for her because she was constantly being made fun of and because i should have told my friend to leave her alone and should have stood up for her.In this situation my options were to help her and stand up for her and actually lose one of my friends or I can stay quiet and not say anything.It is my belief that people bully others because it makes the bully feel good, something that happened to them in the past and they want to let their anger out on someone,they have problems at home currently,they don't like that person,and it is for revenge.I believe they want to hurt their peers because they can see how vulnerable they are and how easily they can bring then down.Sometimes they do it because other people do it,they just want to feel cool,or they feel like they are tough.