In the 60's and 70's, a bare majority of Americans favored capital punishment. But mounting fear of crime, and the cynical manipulation of the death penalty issue by many politicians for their own political gain, led to a shift upwards. In the early books of the Old Testament, one finds standardized views of punishment and death. Ideas of resurrection or return from death were non-existent. Death was the end of one's life.
Death is designated as a 'dead end road'.
Clearly this illustrates God's displeasure with the act of murder. God cursed him from the ground. Whoever kills Cain will suffer a sevenfold vengeance. And the Lord put a mark on Cain, so that no one who came upon him would kill him. God did not kill Cain but punished him by a curse of the soil and banishing him from His presence.