Murder wasn’t something God wanted to see in a community.
Murder attracted a death sentence; manslaughter a lifetime in a city of refuge (until the High Priest died). Neither could be commuted by a any type of payment: neither bribe nor compensation was acceptable. It simply wasn’t possible for the guilty to buy their way back into their community.
The explanation? Murder pollutes the land:
Numbers 35:33-34This will ensure that the land where you live will not be polluted, for murder pollutes the land. And no sacrifice except the execution of the murderer can purify the land from murder. You must not defile the land where you live, for I live there myself. I am the Lord, who lives among the people of Israel.
Was it overstatement? Could murder really pollute the land?
It’s consistent with the way God spoke to Cain about murdering his brother, Abel:
Genesis 4:10-12 What have you done? Listen! Your brother’s blood cries out to me from the ground! Now you are cursed and banished from the ground, which has swallowed your brother’s blood. No longer will the ground yield good crops for you, no matter how hard you work! From now on you will be a homeless wanderer on the earth.
The blood ‘cries out’; the ground won’t ‘yield good crops’ regardless of Cain’s efforts. For Cain, the ground was polluted and useless.