I think that it can be done for different reasons. Sometimes people may just be craving a sense of power. It may be similar to bullying or child abuse. You take advantage of somebody weaker than you to try to fill some void in yourself. I guess some people also do it out of sexual frustration. They feel that nobody will give them what they want. Then they just decide to take it. They may be angry or jealous that somebody else it getting "it" and they're not. They may even "rationalize" that they deserve "it" more. None of these things are actually justifications, just an attempt at an explanation.