The fourth and fifth chapters of Revelation focus on a period of history when God judges sinful humanity. These chapters address the question of who is worthy to judge the whole world. Only a perfectly sinless being could stand in judgment of another being. God is holy and perfect; he has never sinned. He is qualified to judge.
A morally perfect God must judge sin. His perfect justice does not allow sin to go unpunished. So, he sent his perfect, sinless Son to pay the penalty we all deserve.