Genesis 4:10-11 bloodguilt first mentioned
Genesis 4:10–11
[10] And the LORD said, “What have you done? The voice of your brother’s blood is crying to me from the ground. [11] And now you are cursed from the ground, which has opened its mouth to receive your brother’s blood from your hand. (ESV)
Matthew 23:33-39 Acts 5:28
Jeremiah 26:12-14 - (non heathen nation but same principal)
God even requires the life of beasts who shed mans blood (also in Gen 9) - so certainly heathen who shed mans blood.
Genesis 9:6 is noahic covenant but proto or pre civil
Numbers 35:33 key verse. Especially more depraved in the land of the people of who the Lord dwells in their midst but objectively unjust blood shed defiles all lands (and nations, cities and inhabitants as Jeremiah later says)