Justice is defined in God’s nature. It is defined by who He is, not by what we want.
God’s justice is shaped out of His wholeness and His love. By wholeness we mean completeness and goodness; there is no evil or lack in God. He is the complete package of perfection. By His love we mean His perfect desire for us. This is part of what makes Him the only real source of true and perfect justice.
God is knowable but not completely understandable by man, which makes it challenging for us to understand Him and His justice. It is beyond us. Sometimes we don’t understand His actions (or inactions). Our thoughts alone are too low, selfish and imperfect. We need to pursue God; justice is just some of the fruit of that journey. We can know justice by knowing God and trusting His desires for us.