How do we know the commandment 'Thou shalt not kill' applies to our hearts?

In Matthew 5:21-22, Jesus expands upon the commandment 'Thou shalt not kill' to emphasize that hatred and anger towards one's brother make one guilty before God. This illustrates that the commandment is fundamentally about the attitude of the heart, not merely adherence to physical acts. Christ's teaching reveals the true nature of sin as it begins in the heart, proving that everyone has the potential for murderous thoughts without necessarily committing the act. Thus, God’s law addresses our moral failures in attitudes and motivations, underscoring our need for Christ's redemptive work in our lives.