The earth will respond to Cain as if it had personality of its own. So horrific is Cain’s crime that even the inanimate earth will behave as if it were shocked and appalled by what he has done.
Why was the death penalty not applied in Cain’s case? It was not until after the flood that God established the death penalty. But then it was to be carried out by human beings on the basis of human testimony but in the case of Abel’s murder there were no human witnesses. The Lord alone had witnessed Cain’s murderous act, and the punishment he receives from God directly, teaches us that the Lord makes himself the avenger of his people and the judge of all those crimes which are not detected by human justice.