Whoa... that ending is a shocker. After absorbing all his other self had to say about his perfect world and Doom's decidedly imperfect self, all Hell suddenly breaks loose. And is that what it all ties back to? Back in issue #3, which seems like ages ago since this series was interrupted by the pandemic Doom was down in Hell since Taskmaster killed him in issue #2. Mephisto was practically rubbing his clawed hands together in anticipation when Mistress Death intervened and sent Doom back to the land of the living.
This issue seems to come full circle when the series concludes with the death of untold billions, trillions. It's probably irrelevant to Mistress Death. But does this pay off Doom's account with her? I considered that possibility since Doom did seem contrite about his failings being pointed out to him. But perhaps his other more perfect self began to grate on him. Sackcloth and ashes are not exactly part of Doom's wardrobe. So was this payment in full to Mistress Death or just Doom rejecting any attempt to reform? I'm thinking it's a bit of both.