“The story of Cain and Abel thus demonstrates a theme widespread in the Tanakh:the difference between God's will and human conventions, such as primogeniture. In this text, the emphasis falls, however, not on the reasons for God's preference, but on Cain's fatal and culpable refusal to reconcile himself to it"

"Although murder is a capital offense in biblical law(e.g.,Exod.21:12), the LORD yields to Cain's plea and protects him from the fate he inflicted on Abel. The irony is pungent: The man who could not tolerate God's inscrutable grace now benefits from it. A midrash sees Cain as the first penitent and attributes his pardon to his repentance".

From Jewish Study Bible, Tanakh Translation.