Murder is sin (because it sheds human blood).
After they were kicked out of the Garden, Adam and Eve had two sons: Cain and Abel.
In time, Cain grew angry (jealous) of Abel and killed him.
9 Then God said to Cain, “Where is Abel your brother?” He replied, “I do not know. Am I my brother's keeper?” 10 Thus God said, “What have you done? The voice of your brother's blood cries out to Me from the ground. 11 So now you are cursed from the earth, which has opened its mouth to receive your brother's blood from your hand.
(Genesis 4:9-11 OSB)
We have previously established that:
When GOD gives a human life, that person's blood is the fuel that powers their life.
Because GOD creates each human life, each human life belongs to Him. Therefore their blood belongs to GOD also.
For this reason, the Bible often refers to murder as shedding innocent blood.
When a person murders another person, they are taking what does not belong to them.
The shedding of innocent blood is perhaps the greatest offense and sin because it snuffs out life. GOD is life. GOD longs to share the gift of life with others.
Murder is also borne out of hatred. Hatred is the opposite of love. GOD is love.
Murder disrupts GOD's designed order. When GOD creates a life, He wants that person to live (until such time as GOD no longer permits them to live, in this life). Taking that life away is contrary to GOD's designed order.
For all of these reasons, murder is sin.
To help us appreciate the seriousness of murder in GOD's eyes, consider how GOD punished Cain, the first (human) murderer:
12 When you till the ground, it shall no longer yield its strength to you. You will be groaning and trembling on the earth.” 13 Then Cain said to the Lord, “My guilt is too great to be forgiven! 14 Surely You have driven me out this day from the face of the ground; I shall be hidden from Your face; I shall be groaning and trembling on the earth. Then it will happen if anyone finds me, he will kill me.” 15 So the Lord God said to him, “Not so! Whoever kills Cain, vengeance shall be taken on him sevenfold.” Thus the Lord set a sign on Cain, lest anyone finding him should kill him. 16 Then Cain went out from the presence of the Lord and dwelt in the land of Nod opposite Eden.
(Genesis 4:12-16 OSB)
Murder is a big offense to GOD. Thou shalt not kill.