Those who would elevate the "red letter" passages of the New Testament, that is, the words of Jesus, to a special status in an effort to blunt some of the moral demands found in other parts of the Bible need to consider carefully the words of Jesus in this little snippet from the Sermon on the Mount.
You have heard that it was said to those of old, ‘You shall not murder; and whoever murders will be liable to judgment.’ But I say to you that everyone who is angry with his brother will be liable to judgment; whoever insults his brother will be liable to the council; and whoever says, ‘You fool!’ will be liable to the hell of fire. So if you are offering your gift at the altar and there remember that your brother has something against you, leave your gift there before the altar and go. First be reconciled to your brother, and then come and offer your gift. Come to terms quickly with your accuser while you are going with him to court, lest your accuser hand you over to the judge, and the judge to the guard, and you be put in prison. Truly, I say to you, you will never get out until you have paid the last penny. (Matthew 5.21-26)
Jesus is not softening anything here. In fact, he is raising the bar higher than any scribe or Pharisee ever would. It is not enough to refrain from murder; you must also harbor no animosity toward your brother. Don't even insult him and be especially mindful of the eternal peril associated with calling him a fool.
It was lingering animosity that finally got the best of Cain when he murdered his brother Abel. But, if we read Jesus' words carefully, we will find that Cain's downfall was a bitterness toward his brother which had taken root long before he committed his heinous act. "So if you are offering your gift at the altar and there remember that your brother has something against you," Jesus says, "leave your gift there before the altar and go. First be reconciled to your brother, and then come and offer your gift."
Had Cain been in a right relationship with his brother when he offered his gift, God would have received it. But because Cain was not in a right relationship with is brother he was, likewise, not in a right relationship with God. Bitterness bred jealousy, and jealousy manifest itself in Cain's murderous rage. He had offered his gift from a divided and unreconciled heart, thus allowing sin to gain a foothold in his life.
Jesus calls us to a life of perfect love toward God and one another. We cannot be in a right relationship with God unless we are in a right relationship with one another. An inward orientation toward anger and bitterness will ultimately manifest itself in outward actions which cause strife, division, and even death. The life of holiness begins with an inward reorientation toward love, unity, and peace.