I personally served in the military and had to come to terms with what I believed was the case surrounding killing. Here is what I have concluded.
Exodus 20:13
13 Thou shalt not kill.
Deuteronomy 5:17
17 Thou shalt not kill.
The word "kill" in Exodus 20:13 and Deuteronomy 5:17 means "murder." The proper translation is THOU SHALT NOT MURDER. The most profound interpretation of this is by Jesus Himself in Matthew.
Matthew 19:16-20
16 And, behold, one came and said unto him, Good Master, what good thing shall I do, that I may have eternal life?
17 And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.
18 He saith unto him, Which? Jesus said, Thou shalt do no murder, Thou shalt not commit adultery, Thou shalt not steal, Thou shalt not bear false witness,
19 Honour thy father and thy mother: and, Thou shalt love thy neighbour as thyself.
20 The young man saith unto him, All these things have I kept from my youth up: what lack I yet?
The Hebrew word (ratsach) and the Greek Word (phonenō) which are used in the Sixth Commandment both mean "murder."
The Hebrew language has a general word for killing (muwth, meaning "to cause to die") the Greek language has a general word for killing (apokteinō), but these general terms for killing are not used in the Sixth Commandment. Instead very specific words are used which forbid MURDER.
Murder involves killing unlawfully with premeditated malice. It involves a deliberate, planned, pre-mediated attack against a fellow human being for the purpose of taking his life for reasons that are purely sinful.
The Bible makes a clear distinction between someone who kills a person accidentally without ever having hated the person (Deut. 19:4), and someone who murders a person by lying in wait for him (Deut. 19:11–13). The latter is an example of a carefully planned and premeditated murder motivated by hatred.
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.
Warfare, as terrible as it it, is sometimes necessary in this world, the greatest battlefield slaughter of all time will take place in the future at Christ's second coming. This final battle will led by the Lord Himself at Armageddon (Revelation 19:11-19).
God has directly killed or commanded the killing of millions of people in the Old Testament for various reasons, all righteously.
In the end, there are countless opinions on this subject, but there is a clear biblical answer to the question as well. We all have to make peace with our decisions, but from the biblical perspective, this issue is resolved from the Lords own words Himself.