God revealed to Moses that blood that was shed by murdering someone would pollute the land unless the Israelites dealt properly with that bloodshed:
Num 35:30 Whoso killeth any person, the murderer shall be put to death by the mouth of witnesses: but one witness shall not testify against any person to cause him to die.
31 Moreover ye shall take no satisfaction for the life of a murderer, which is guilty of death: but he shall be surely put to death.
32 And ye shall take no satisfaction for him that is fled to the city of his refuge, that he should come again to dwell in the land, until the death of the priest.
33 So ye shall not pollute the land wherein ye are: for blood it defileth the land: and the land cannot be cleansed of the blood that is shed therein, but by the blood of him that shed it.
34 Defile not therefore the land which ye shall inhabit, wherein I dwell: for I the LORD dwell among the children of Israel.
Based on this sobering revelation, how polluted in the sight of God must all the countries of the world today be because of all the blood that has been shed by murderers in those countries that has not been properly dealt with by punishing the murderers properly!