It is a question you should be able to answer and it sounds like your overthinking it.
There must be something that has happened to you or a family member that in your life that was unfair. A parent being laid off, relative dying of cancer, your car being broken into, I imagine your at least 20 years old and something in your personal life or something the government or other organization has done upsets you.
You could even use 9/11 as an example or the school children in Connecticut that were shot.
The essay doesn't have to be great, but if you are going to attend law school you should have some sort of passion about something and I am sure you do, but don't overthink it.
Lawyers and law students have a tendency to overthink issues making them far more difficult than they need to be and I know I did the same thing when I was in your shoes and even in law school, but it really slows you down.