Murder was wrong before it was prohibited in the law of Moses (Gen.4:6-15, 9:5-6). Before the commandment prohibiting murder was codified in the Law of Moses, it was in God’s law written on human hearts (Rom.2:15). In Matthew 5:21-48, Jesus includes it in what we may call the Law of Christ (1Cor.9:19-23). Jesus fulfilled the law of Moses (Mat.5:17), brought it to an end as a rule of life, and gave his “version” of the law of God for people under the New Covenant.
Jesus is the new lawgiver. In Matthew 5:21-48, He references the law of Moses by saying six times, “You have heard that it was said to those of old” (or something similar). He follows that with, “But I say to you” to indicate a contrast or addition to what Moses said. According to the law of Christ, not only are people who commit murder in danger of judgment, but someone who is angry with his brother (Mat.5:21-26). Ultimately, anger may lead to murder, so Jesus demands quick reconciliation with a brother or adversary. We must not allow anger to fester, but seek swift reconciliation with any brother or sister who has something against us.