"If anyone sins in that he hears a public adjuration to testify, and though he is a witness, whether he has seen or come to know the matter, yet does not speak, he shall bear his iniquity;
This verse illustrates the importance of 'justice' to God. A person who knows of a wrong done but fails to act when they can ensure justice is done, that the offender receives judgement by the proper authority; this person will not escape their own justice. They will be seen by God who will ensure that justice is done because he has the sight to see all things and the power to bring rightness to the universe he created.
Because we know that God requires justice to be done, we can know that it will be done, even when it doesn't seem that way to us. Nobody can get away with their wicked deeds because even after this life they will face God to receive his judgement on their lives.
The question of why God would allow pain and suffering is also answered in this: he is 'just', so the pain and suffering are either right and fair, or he will bring justice to those who caused that pain, or both. So if you are suffering, or have suffered, seek God's justice in that unpleasant experience and be ready to trust in his wisdom in case you can't find a reason (learning to trust him or to see your own sin might be the reason).