The law says "thou shalt not kill", but Jesus adds that even if we are angry with our brother or sister we are liable to judgement. Even more radically, if our brother or sister has reason to be angry with us, we also need to heal that division before we come to the altar. Relationship is a two way street.
Matthew 5:20-26
Jesus said to his disciples, 'For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven.
‘You have heard that it was said to those of ancient times, “You shall not murder”; and “whoever murders shall be liable to judgement.” But I say to you that if you are angry with a brother or sister, you will be liable to judgement; and if you insult a brother or sister, you will be liable to the council; and if you say, “You fool”, you will be liable to the hell of fire. So when you are offering your gift at the altar, if you remember that your brother or sister has something against you, leave your gift there before the altar and go; first be reconciled to your brother or sister, and then come and offer your gift. Come to terms quickly with your accuser while you are on the way to court with him, or your accuser may hand you over to the judge, and the judge to the guard, and you will be thrown into prison. Truly I tell you, you will never get out until you have paid the last penny.
(New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org)