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The rivalry between Dante and Vergil just becomes heartbreaking by the end of the game. Throughout the game, the two have worked together to save the world from Mundus, and the game built up a very believable friendship between the two. Seeing it all break down at the end was like a shot to the gut, even if it was a Foregone Conclusion to some.
Even worse considering it happened because they each had differing ideas as to how to protect and value the humans and were both unwilling to listen or try to appeal to each other in expressing their plans.
Afterwards, Dante is so broken that he doesn't know who he is anymore or what he's supposed to do. This leads to a Heartwarming moment as Kat insists that he's Dante, nothing more and nothing less.
The DLC destroys any last hope of Vergil and Dante reconciling their differences.
Mission 4 of Vergil's Downfall was particularly saddening in that Vergil saves his mother's soul, who was being held captive by an Imprisoner, only to tell her that he killed Dante, neither knowing it was actually a doppelganger, in a very spiteful manner and then coldly leaves her to mourn the death of one of her sons.