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 render Exodus 20:13 like the New King James Bible.
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).
Ancient Israelites, under the Old Covenant, 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). He 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 (Matthew 5:21 - 26).
The apostle John elaborates on this by writing that to hate someone is the same as murdering them. He states, "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).
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. The more we walk with God and do his will, the more we will fulfill "Thou Shall not Kill."