Justice is the supreme Judge. Here we see the effects of Karma, the idea that everything we do today has either a positive or negative effect upon our future. Every dark thought or deed weighs against us, balanced out (hopefully) by every good deed and loving thought.
We can be angels and we can be demons, it is our choice. We shape our futures by what we do and think now. If we stop blaming other people, or forces, if things go wrong then we take control of our own lives. Justice shows us that the natural cycles of the Universe will reflect back upon us exactly what we expect - so expect joy and happiness!