Matthew 5: 21-22 says “You have heard that it was said to those of old “You shall not murder, and whoever murders will be in danger of the judgment.” But I say to you that whoever is angry with his brother without a cause shall be in danger of the judgment.” Since God never changes, He never changes His rules. So did Jesus add to this law, or was He saying that God always meant that law to mean that?
The Lord’s objective in this portion of the Sermon on the Mount was to explain what He meant by saying, “Unless your righteousness surpasses that of the Pharisees and the teachers of the Law, you will certainly not enter the Kingdom of Heaven” (Matt. 5:20). They believed that by not killing anyone they were keeping the commandment prohibiting murder. Jesus explained that in God’s opinion even being angry with a brother was a violation of the commandment because anger is the first step in the process that sometimes leads to murder.
Then He went on to give other similar examples to show that it isn’t just behavior that makes us subject to judgment, but also motive. These weren’t new laws, just a more comprehensive explanation of the old ones.
Since no one can completely avoid being angry or having lustful thoughts, etc. no one can keep the commandments. The Pharisees were better at controlling their behavior than just about anybody, but even perfect behavior can’t meet God’s standards. That’s why we need a Savior.