The main difference between Justice and Humanitarianism is that they affect different groups of people.
Justice is a concept of moral correctness, based on any number of philosophical and religious foundations. Justice requires that people are treated the same from the single moral standpoint, and that transgressors are punished to give fair, or just, response to their transgressions. While it covers all people, legal Justice only applies directly when laws are broken. Even without a perfect society, the application of Justice to everyone would support moral fairness to all.
Humanitarianism is a concept of sympathy towards humans; the idea is that altruistic tendencies are inherently moral because Humankind is important. Humanitarianism applies to all people in every situation, rich or poor, high class or low, foreign and domestic. This broad definition allows a basic level of human kindness to be practiced regardless of the situation.
Therefore, Justice applies in practice to fairness in response and between like situations, while Humanitarianism applies in practice to acceptance of everyone, all the time.