Retributor.com, Retributory.com, and TheRetributor.com -- Retributor, Retributory, and The Retributor
These gaming-style domains would be very appropriate for a gaming site and a fictional character hell-bent on retribution.
Retributor is a dictionary word; according to Merriam-Webster, a retributor is "one that exacts retribution or pays retribution."
Retributory, also a dictionary word, is the adjective form of Retributive.