ANSWER: The commandment "thou shalt not kill" (found in the KJV Bible translation of Exodus 20:13), also listed as "thou shall not kill" is better understood in the New King James Version Bible.
"You shall not murder." (Exodus 20:13, NKJV throughout)
Most modern translations of the Bible rendered it this way.
According to the Bible not all killing is murder. Murder is the unlawful taking of a human life. The command not to murder applies to human beings and not to animals. God gave animals to mankind for his use (Genesis 1:26-30; 9:1-4). But, this does not mean that humans have the right mistreat animals and the environment (Genesis 2:15; Deuteronomy 22:6-7; 25:4; Proverbs 12:10).
Under the Old Covenant ancient Israelites were allowed to kill other humans under very special circumstances such as punishment for certain sins, for example, murder (Exodus 21:12-14, Leviticus 24:17, 21) and adultery (Leviticus 20:10, Deuteronomy 22:22-24). God also allowed the Israelites to engage in warfare and even gave them instructions about waging war (Deuteronomy 20:1-20). God also recognized that humans might accidentally kill each other, and he made provisions for this (Numbers 35:9-34; Deuteronomy 19:1-13).
The primary reason God hates murder is that out of all creation, only human are made in his image and likeness (Genesis 1:26-27; 9:4-6). Even before the codification of the Ten Commandments at Mount Sinai, the murder of other human beings was wrong (Genesis 4:8-12; 4:23-24; 9:4-6; Exodus 1:16-17). While on earth, Jesus spoke out against murder (Matthew 5:21-26; Mark 10:17-19). We also see in the writings of Paul (Romans 1:18, 29-32; 13:8-10; Galatians 5:19-21), James (James 2:8-11; 4:1-3), Peter (1 Peter 4:15-16) and John (Revelation 9:20-21; 21:7-8; 22:14-15) that murder is wrong.
In Matthew 5:21-26 Jesus amplifies the meaning of the sixth commandment "thou shall not kill." He brings out that to commit murder means more then just killing someone, it means having an angry and unforgiving attitude towards them.
"You have heard that it was said to those of old, 'You shall not murder, and whoever murders will be in danger of the judgment.' But I say to you that whoever is angry with his brother without a cause shall be in danger of the judgment. And whoever says to his brother, 'Raca!' shall be in danger of the council. But whoever says, 'You fool!' shall be in danger of hell fire.
"Therefore if you bring your gift to the altar, and there remember that your brother has something against you, leave your gift there before the altar, and go your way. First be reconciled to your brother, and then come and offer your gift. Agree with your adversary quickly, while you are on the way with him, lest your adversary deliver you to the judge, the judge hand you over to the officer, and you be thrown into prison. Assuredly, I say to you, you will by no means get out of there till you have paid the last penny." (Matthew 5:21-26, NKJV)
The apostle John elaborates on this by writing that to hate someone is the same as murdering them.
"Whoever hates his brother is a murderer, and you know that no murderer has eternal life abiding in him." (1John 3:15, NKJV).
Murder like all sin, beginnings in the human mind (Matthew 15:18-19; Mark 7:20-23) it starts as a thought, in this case hatred, which leads to the action of murder (James 1:13-15; 4:1-3). The opposite of hating someone is loving them, we should even love our enemies (Matthew 5:43-48), seeking not revenge, but looking for ways to help them (Romans 12:17-21).
As we have seen, under the Old Covenant God allowed humans to kill other humans under certain circumstances. But what about today, and those who are now under the New Covenant, should we participate in the execution of a murder or an adulterer? Should Christians involve themselves in warfare? I would argue that Christians should not participate in such activities because the New Covenant is a covenant of life, not of death (2 Corinthians 3:4-11). Under the New Covenant Christians do not execute people for sinning. The most drastic steps anyone can take against an unrepentant Christian are withholding brotherly fellowship from him until he repents (1 Corinthians 5:9-13; 2 Thessalonians 3:14-15); and delivering or handing him over to Satan for spiritual correction (1 Corinthians 5:1-5; 1 Timothy 1:18-20).
Under the New Covenant a change occurred in the meaning of 'thou shalt not kill.' Just as Christians are to no longer execute sinners, so they should not wage carnal war, but spiritual warfare (John 18:36; 2 Corinthians 10:1-6; Ephesians 6:10-18; 1Timothy 1:18-20; 6:11-14; 2 Timothy 2:3-5; 4:6-8). Christians must be peacemakers (Matthew 5:9, Romans 14:19), forgiving those who do them harm (Ephesians 4:29-32; Colossians 3:12-14; Matthew 6:9-15; Mark 11:25-26), treating their enemies with love (Luke 6:27-36) and not seeking revenge (Romans 12:17-21; 1Peter 3:8-12). Hatred, which is the same as murder (1John 3:15), is unforgiving, vengeful and hostile towards one's enemies.