With Platinum Angel safely on the battlefield, your life total becomes nothing more than a resource to be abused, allowing you to bypass the usual safety limits of the game. When you cast Volcano Hellion, its enters-the-battlefield trigger lets you assign a massive, arbitrarily large amount of damage to both yourself and Spitemare simultaneously. Because that unpreventable damage is dealt directly to Spitemare, its reflexive damage-dealing trigger fires right back, taking the massive number you chose for Volcano Hellion and redirecting it straight to an opponent's face to close out the game in one fell swoop.