If Cain exists, that also means there's a good chance Lucifer does as well. How might the "fallen one" interact with Cain? Unless Cain's already done him in and sort of taken over his portfolio...
I could see Cain using his Sevenfold curse in creative ways to strike back at the gods. For example, investing in the bank held by the God of Riches and then enticing one of the bankers to steal/depreciate his investment, causing his sevenfold curse to manifest in destroying the bank's wealth and rendering the God of Riches powerless for his lieutenants to destroy.
To bring about an apocalypse, Cain would need to assault the pillars of society - performing acts such as destroying the financial system, disrupting local law enforcement and/or military, and bring governments to ruin. He could present himself as a freedom fighter or terrorist, working against corrupt systems held in place by uncaring gods who disguise their true nature as the prominent politicians or illuminati who pull societies strings. Perhaps that what the gods are - forces of culture and order attempting to impose their will on humanity. As long as that sociological order survives, so does he. If he and his associates can utterly and permanently remove that power structure, he can finally rest.
<EDIT> some ideas?
Ares, controls the Pentagon and directs the Afganistan war
Thor and/or Zeus, leading the charge to bring climate control in check
Athena, leading the feminist movement to bring equality to all genders
Hades, in charge of the CDC, controlling the flow of information to forestall death