I always thought Dante got more messy after DMC 1 because he reached his goal of vengeance. After DMC 1, he basically have nothing to drive him forward, leading him to fall into depression. There was Trish at first, but she eventually left because she needed to forge her own life and could not stay with Dante with his mother appearance.
So Dante spended many year alone in his office, got more and more depressed over time and realized that vengeance doesn't bring peace. But at this point, he was way too powerful and this alienated him to society. Rules and society are made for common people, not for overpowered being like him. So he isolated himself in his office and didn't cared about everything. The only person that could stay with him was his brother,, but he failed him in DMC 3, leading to his downfall in DMC 1. To me, that's why Dante cared so much about Nero. He is the last remnant of Vergil to him, and Dante wanted better for him. That's why he took on Urizen alone. He thought just witnessing Vergil's state would harm Nero, and thus tried his best to shoo him away, until Nero showed him that he didn't needed to stay alone. Vergil was right here, and Nero was ready to take after Dante. So Dante left with the only person he could be with, his brother.
I also think that I am overthinking, and that the DMC team has no clue about the story.
So Dante spended many year alone in his office, got more and more depressed over time and realized that vengeance doesn't bring peace. But at this point, he was way too powerful and this alienated him to society. Rules and society are made for common people, not for overpowered being like him. So he isolated himself in his office and didn't cared about everything. The only person that could stay with him was his brother,, but he failed him in DMC 3, leading to his downfall in DMC 1. To me, that's why Dante cared so much about Nero. He is the last remnant of Vergil to him, and Dante wanted better for him. That's why he took on Urizen alone. He thought just witnessing Vergil's state would harm Nero, and thus tried his best to shoo him away, until Nero showed him that he didn't needed to stay alone. Vergil was right here, and Nero was ready to take after Dante. So Dante left with the only person he could be with, his brother.
I also think that I am overthinking, and that the DMC team has no clue about the story.