Why'd the Bible Tell Us of 'the Mark of Cain' If Cain's Line Died in the Flood?

[mythociate]

The Bible tells us that Noah & his sons were the only people left to repopulate the entire world. Why then did the Bible-compiling Councils think it so important to remember the mark of Cain---the sign that sevenfold vengeance would be taken on any who 'exterminate' him.

I think it was (being 'legalist') to 'set precedent' for the fact that vengeance (taken consciously by humans) is more 'an addiction' than 'a cure'---look at Tim Burton's BATMAN ... Jack "The Joker" Napier's death was just the beginning for him, leading him down a never-ending series of violent confrontations (disguised as 'serving the people of Gotham).

Luckily, Christ(opher) Nolan delivered Bruce Wayne from the decay-pattern of the condition 

But the Bible's warning remains, that--no matter how "justified" we may think we are in dealing 'blood for blood & tooth for tooth'--'conscious performance' of the vengeance brings with it the addiction/responsibility of rendering more-&-more vengeance ... possibly making the performer the host to a 'vengeance demon' (like Xander's girlfriend/wife on TV's Buffy the Vampire Slayer).