Anger.

"You have heard it said long ago, 'Do not murder.' and whoever murders will be subject to judgement. But I say to you, anyone who is angry with his brother without a cause will be in danger or judgement. If you call someone an idiot, you are in danger of coming before the court. And if you call him a fool, you are in danger of hell (eternal destruction)." Matthew

Jesus was quoting the Ten Commandments where it says, "Thous shalt not murder." He is saying there is more to this commandment. Anger leads to murder; so watch out for anger. If we move from anger to calling people names it can escalate even farther. The next step is hitting them or beating them.

What our Lord seems here to prohibit, is not merely that miserable facility which some have of being angry at every trifle, continually taking offense against their best friends; but that anger which leads a man to commit outrages against another, thereby subjecting himself to that punishment which was to be inflicted on those who break the peace. (Clarke's Commentary on the Bible.)

Anger is such a powerful emotion. I've learned over the years that when I feel anger growing inside to start praying right away for patience.  I still slip up once in awhile, but God tells me when I do. He works with us and forgives us.

One time, many years ago, I lost my temper at the lady at a dress shop who was rude to me and wouldn't let me return defective merchandise. I walked to my car and God told me I had to go back and apologize, which I did. Let me tell you, it is a good lesson! God was right of course, it was wrong of me to be nasty to a child of his, to a fellow human being who was maybe going through hard times. There was no excuse. Jesus didn't yell at the people who crucified him, how could I yell at someone who wouldn't take a pair of jeans back?