Some people waste their lives hating others instead of improving their own lives, placing blame for their own failures on others. It is human nature to need someone to blame sometimes to vent frustration, but once this basic need is filled, one needs to let it go. Life is too short to spend it on things like revenge that don't make any difference in the end. Such activities are often likely to end up causing more harm to oneself than the intended target.
Often such feelings are based on a misunderstanding of the whole situation; angry people rarely look at problems rationally, become paranoid, and take things out of context. I have had this happen to me personally a couple times over the years. Advice such as this is often given and rarely heeded; indeed one may need to experience both sides before really understanding it, and some are so focused on hate they can't let it go even with experience.