Long before God commanded the sixth commandment of the original Ten commandments to not murder; it was an official law pretty much well known everywhere that God commanded humans to not murder innocent humans.
The first recorded murder: The murder of Able by Cain
Unlike many laws before the old covenant this law not only strongly suggested as a law but was one of the first laws openly commanded by God himself to obey and punishment to be given if it was not obeyed. The first murder ever recorded in the Bible (and in history), was the murder of Able by his own brother Cain: Able and Cain were the first known children of Adam and Eve that they had shortly after they broke God’s command for them and were banished from the garden of Eden. They were also the first Adamites (that is descendants of Adam) that were born with a natural understanding right from wrong and had to choose with their own freewill wherever to obey God or do what they wanted (Evil: this choice was also known as the Adamic covenant). So, under God’s presents Adam and Eve raised Cain and Able and when both got old enough, they got involved in farming: the older brother Cain choose to grow plants (as commanded by God for a Adamite/Human man to do under the Adamic covenant) but Able choose to herd sheep (most likely for more clothes to be made from their wool: another command for God):
Genesis 4:2
“Now Abel kept flocks, and Cain worked the soil (grew plants).”
And eventually both of them did what they were told as another command in the Adamic covenant (either told by God himself or indirectly through their father Adam about this law) to offer as a sacrifice the very best of their produce (for Cain that would be the finest fruits/vegetables/grains he grew and for Able that was the fat from one of the murdered firstborn sheep in his heard):
Genesis 4:2-4
“In the course of time Cain brought some of the fruits of the soil as an offering to the Lord. And Abel also brought an offering—fat portions from some of the firstborn of his flock.”
When they began to offer these offerings of their produce God approved of Able’s offering of the fat from his killed firstborn sheep: he had offered his very best and God was pleased with his obedient sacrifice to him. However, God did not approve of Cain’s offering of some of his grown fruit: why it's not openly said what exactly was wrong with Cain’s offering, something was not correct about what or how Cain offered this offering, and it made God displeased at his disobedience to him:
Genesis 4:4-5
“...The Lord looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor.”
Once Cain saw that God did not approve of his offering to him he became very angry at God’s rejection of him over Able and became angry and began to frown (called ‘his face being downcast’).
Genesis 4:4-5
“...So Cain was very angry, and his face was downcast (sad).”
God decided to talk to Cain about his anger towards his rejection of his sacrifice: There was no reason for him to be angry about this and frown; Cain apparently knew what he did wrong on his offering and all he needed to do was do a correct offering (and also all the other commands he gave him to follow) and God assured him he would a prove of it and accept him like his brother Able:
Genesis 4:6-7
“Then the Lord said to Cain, ‘Why are you angry? Why is your face downcast? If you do what is right, will you not be accepted?’”
But God warned him to not choose to do evil and rebel against him: he had to overcome his desire to do evil/what he wanted in his anger and instead choose to do right by making a correct offering and obeying anything else he told him:
Genesis 4:7
“...But if you do not do what is right, sin (evil) is crouching at your door; it desires to have you, but you must rule over it.”
But sadly Cain choose to give into his anger in jealousy of his brother Able and decided to beat him up (hurt him) in his anger; so he tricked him but offering to go into a field so he could beat him up in secret: so he asked his brother to go into the field with him; so Able went walking into the field; once there Cain beat him up:
Genesis 4:8
“Now Cain said to his brother Abel, ‘Let’s go out to the field.’ While they were in the field, Cain attacked his brother Abel…”
but wherever he intended on it or not his brother died from the inquires and bleeding to death: Cain has murdered his own brother out of jealously. God saw all of this and he came and questioned Cain asking him where his brother was:
Genesis 4:8
“...and killed him.”
Genesis 4:9
“Then the Lord said to Cain, ‘Where is your brother Abel?’ ”
Cain tried to lie to God and tell him he did not know where his brother was; was he responsible for keeping up with his brother?
Genesis 4:9
“’I don’t know,’ he replied. ‘Am I my brother’s keeper?’”
Genesis 4:10
“The Lord said, ‘What have you done? Listen! Your brother’s blood cries out to me from the ground.’”
Cain had chosen to do evil in his anger: he choose to murder his brother and thus reject him and do evil, which God defines here as something wrong thus evil, and decides to curse Cain for his sin of murder:
Since his brothers blood feel into the dirt, he cursed Cain to never be able to grow anything ever again and would be forever banished from the land near the garden of Eden where he and his parents lived for his horrible sin of murder and would never be at true peace:
Genesis 4:11-12
“Now you are under a curse and driven from the ground( both the land around Eden and from growing anything from the dirt on the ground), which opened its mouth to receive your brother’s blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth.”
Genesis 4:13-14
“Cain said to the Lord, ‘My punishment is more than I can bear. Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me.’”
But God had some mercy on Cain; even though he was going to be horribly cursed God said he would make sure no one would kill him because he murdered: if anyone did God would curse them many times more for killing him as well:
Genesis 4:15
“But the Lord said to him, ‘Not so; anyone who kills Cain will suffer vengeance seven times over.’”
To let other humans (possibly also Adamites) know he put a special mark on Cain’s body so if another human found him and wanted to kill him they would see the mark on him and know to leave him alone:
Genesis 4:15
“Then the Lord put a mark on Cain so that no one who found him would kill him.”
So Cain was forever banished from the land around Eden where his parents lived and choose to live in a new land in the East from the land of Eden that he called Nod (which means ‘wandering’) never to return to his homeland with his parents and his new siblings and especially where God was living with his Adamite human creations:
Genesis 4:16
“So Cain went out from the Lord’s presence and lived in the land of Nod, east of Eden.”
So, seeing God’s reaction to the first recorded murder, we can clearly see murdering is evil and thus a sin almost as soon as the Adamic covenant was established; so obviously it would make sense if God had before this first recorded murder or afterwards commanded to Adam and Eve and their later children to not murder other humans.
About 1,000 years from this first recorded murder as Adamites began to choose to reject God and do evil many of them began to get very violent and murdered often;
Genesis 6:5-7
“The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time.”
Genesis 6:11-12
“Now the earth was corrupt in God’s sight and was full of violence.”
(notice the connection with evil thoughts and violence/murder here)
God was so horrified by all this violence and murder from rebellious Adamites he even got to where he regretted making them!
Genesis 6:6
“The Lord regretted that he had made human beings on the earth, and his heart was deeply troubled.”
He decided to kill all these corrupted human in a massive flood because of their evil and violence:
Genesis 6:13-17
“So God said to Noah, ‘I am going to put an end to all people, for the earth is filled with violence because of them. I am surely going to destroy both them and the earth.…I am going to bring floodwaters on the earth to destroy all life under the heavens, every creature that has the breath of life in it. Everything on earth will perish. ’”
So God killed all these violent evil Adamites leaving only peace loving Noah and his family alive, then when he was making the new Noahic covenant with him and his family he openly states officially for the first time how murder would not be allowed of not just (but especially) human lives but also any animal lives (apart from murdering them for clothes/food or even self defense) and he would hold the killer accountable for now on for why they killed that human or animal:
Genesis 9:5-6
“...for your lifeblood (that is shed in hurting/killing you: murder) I will surely demand an accounting. I will demand an accounting from every animal. And from each human being, too, I will demand an accounting for the life of another human being.”
God especially said the murder of another human being was a very serious crime: because unlike animals, since humans were created in the image of God himself, their lives were very valuable in comparison; so a murder of a human life would carry a punishment of the murderer being killed by other humans in return (they were to be killed for their crime):
Genesis 9:6
“Whoever sheds human blood, by humans shall their blood be shed; for in the image of God has God made mankind.”
So, if this law was not understood in the former Adamic covenant (most likely it definitely was) it definitely was made clear that murder was unacceptable in the new Noahic covenant. And even when almost all Noahites pretty much rebelled against God and began to forget his Noahic covenant by worshiping false man-made Gods, many of these now Noahic nations/peoples still kept the law of not murdering for humans (except in human sacrifices to their gods) and many had national laws for citizens to not murder and if they did often the punishment was death (killing the murderer) just like God told them to do: in this way they obeyed God’s command regarding murder.
So, the law of not murdering was well known and commanded by the time God made the Old Covenant.