Did I say that our last passage was the most difficult one in the Sermon on the Mount? After looking at this one I feel like taking that back.
Jesus says that being angry with your brother is bad, calling him "Raca" is badder, and calling him "fool" is baddest. I don't know how to understand the Aramaic word "raca" so I'm going to skip commentary on that. Let's move to the idea that calling your brother "fool" is an offense deserving the most severe punishment.
The thing I don't get is I recall lots of times in the Bible when Jesus or Paul calls someone "fool" or "foolish." Paul says, "You foolish Galatians!" (3:1) But then I guess he uses the word anoetos, which is different from the word moros that Jesus warns us against using. Paul calls a Corinthians dissenter "fool" (1 Cor. 15:36) but again that is a different word (aphron). James calls his imaginary debating opponent a "foolish fellow" (2:20) but that is the word kenos. Perhaps in those days moros was a more profane term than the others? However, this is interesting: Jesus himself calls the scribes and Pharisees moroi (the plural of moros) in Matthew 23:17 when he says, "You fools and blind men..." So how come he gets to say it and not us? Perhaps it's because he alone is the judge of men's hearts and he doesn't want us to be setting ourselves up as judges too?
From what I can tell, the most common usage of moros and its related forms in the New Testament is to describe how the world looks at the things of God as foolishness, and how God in turn views the wise men of this world as fools (Romans 1:22; 1 Cor. 1:18, 20, 21, 23, 25, 27; 3:18, 19; 4:10). So apparently it is a word that's used to mean the polar opposite of divine wisdom. Could it be that it implies someone is given over to a damnable foolishness? If that's the case then I can see how wrong it would be to go around calling your Christian brothers and sisters that. I'm not a native Koine Greek speaker, but that's the best I can do to make sense of it.
Now what's with all this talk about courts and judges? To me it almost seems like a parable. You know that you are guilty before the court if you insult your brother ("raca", "fool" etc.). You may think you can make up for it by asking God for forgiveness at the altar, but that doesn't get you off the hook. You have to go reconcile with your brother first, then you can safely approach the altar. It's kind of like this. You and your brother are both on your way to the altar to face the Judge. Bringing your offering to the altar before you reconcile with your brother is like going to face a judge in court before you have reconciled with your opponent at law. You are going to walk straight into the presence of a judge with uncleared charges hanging over your head. You might as well be walking straight into a prison cell. So stop. Talk to your opponent. Get things cleared up with him. Now you can to go before the judge safely. Treat coming before the altar and the Judge of hearts in the same way.
While I'm somewhat bothered by the harshness of this teaching (prison, hellfire, etc.), I also get the feeling that it is a polemic against the scribes and Pharisees and anyone else who would reduce righteousness to external law-keeping. Jesus is saying that it's not enough to refrain from murder, but to refrain from committing spiritual murder with your heart and mouth. You cannot plead your external righteousness before God ("I didn't murder anyone therefore I kept the sixth commandment!") because he will judge your very thoughts, words and motives. Jesus' teaching is harsh like a hammer blow, but maybe it's necessary to strike forcefully when hearts are hard. He has to shatter you first before he can build you back up again.