Koheleth Rabbah also says that Kayin had 100 sons, enough to populate a town.
There is ambivalence whether Cain is guilty of murder or manslaughter (no precedent, he got angry, Abel provoked him, or even Hashem provoked him, etc), and his teshuva is half-baked: he demands protection at the end instead of pleading for it. So, he gets half-forgiveness: lives a long life, but is eventually killed in a similar (inadvertent-sic!) manner, not buried.
Also, in an analogy to inadvertent killers, it may be appropriate for him to reside in a city of refuge rather than travelling around forever.
legal reasoning source:
Irene Merker Rosenberg, Yale L. Rosenberg CAIN ROSE UP AGAINST His BROTHER ABEL AND KILLED HIM: MURDER OR MANSLAUGHTER?
they are quoting ALAN M. DERSHOWITZ, THE GENESIS OF JUSTICE 16 (2000) that I did not look up yet.