When Jesus said that he didn’t come to abolish the law but to fulfill it, one thing (among others) which he might've meant was to reestablish the true meaning of the Law. The command not to murder is one of the Ten Commandments, and he certainly didn’t come to correct the law itself. But what he did intend to do was correct our misunderstanding of it.
People would hear the command and think “Well, I’ve never pulled a knife on someone, so I’m fine.” Not so fast. What Jesus is saying here is that God is concerned with our heart attitudes as well as our physical actions. There was never a murder committed with the hands which didn't originate in the heart.
But we need to clear up some misunderstandings here. Is Jesus saying that hatred of someone is just as bad as physical murder? I don’t think so. There are degrees of punishment in hell, and there are levels of accountability, so there are some sins which are worse than others. Any sin is enough to send someone to hell, but not all sin is equal in God’s eyes. The thing we need to keep in mind is that God takes any sin much more seriously than we do. Any sin, any failure of perfect obedience on our part is--in the words of R.C. Sproul--cosmic treason.
But what we need to do, instead of just being concerned about physical action is to constantly reexamine our attitudes towards the people around us. The word Raca is an Aramaic term of contempt, roughly equivalent to calling someone an idiot. And of course Paul called the Galatians foolish, so it’s not so much the term itself that Jesus is warning against. I believe that he’s warning against disregarding the image of God found in every human being. It’s the contempt in your heart which you mutter under your breath when someone cuts you off. It’s truly desiring harm to come to someone who's harmed you. It even extends to squabbles within the church. When a dispute arises, we need to work for reconciliation decisively and quickly, knowing that we need to do this before we dare to come into our Father’s presence in worship.
So you see, it’s much more than merely not hating someone. The standard is much higher than merely avoiding fantasizing about harming someone. God wants us to recognize that every person we meet--no matter how boring or annoying--is stamped with his image, and is precious in his eyes. Only then can we claim that we’re following these instructions. I don’t claim to perfectly obey it, but with his grace I’m trying. How about you?
Lord Jesus, help me to see that person not with my own self-interest, but with your eyes of love, compassion, and mercy. They're precious to you, valuable enough to die for. May my words, deeds, and thoughts reflect this.