How do we know that anger is a serious sin in God's eyes?

In Matthew 5, Jesus clarifies that the command 'Thou shalt not kill' also applies to anger without cause. He states that anyone who is angry with his brother is in danger of the judgment, demonstrating how serious the issue of anger is in the framework of God's law. This teaching shows that God evaluates our hearts and intentions, not merely our outward actions. The commandment's broader application reminds us that sin originates in the heart, which the law reveals as sinful and in need of grace.