The soul was created as a recipient, capable of receiving G-d's light. In a world run on judgment, a person's sin would be enough to incorporate evil into his soul and annihilate him. Once G-d added mercy, however, the evil became an externalized parasite and a single sin no longer leads straight to annihilation. Now that evil is a separate entity, it tries always to expand to assert its existence. The evil will begin to surround that person and bring him great pain and suffering. By allowing the evil to expand, it actually goes on to destroy itself and the soul can be purified. Gain understanding of this mechanism and how it protects us and is the ultimate mercy from G-d.