Why would God threaten to avenge a murderer sevenfold (Gen 4:15)?

There are a few reasons. First, since God took justice into his own hands, man was not to interfere; that was, in that very early age, for God alone. The sentence has already been declared and executed. A further punishment—in the form of vengeance approved by God—would be unjust simply because it would be an additional punishment. God would have taken a life for a life if that had been his sentence. Since he did not, that was not his will. Any further bloodshed would, therefore, not only constitute a new and unjustified act of violence, it would usurp God’s own prerogative of doing justice.