Kat wasn't even a love interest, and thinking Dante did everything just for Kat is really misguided, but considering you went in thinking that, it's not surprising. Dante helping humanity was a result of his feelings that everyone should be free, and after seeing how humanity suffered much worse than even he did, Dante got his act together. He went most of his life not knowing what he really was, and when he was given the chance to learn, he ended up getting used by the one person he thought he could trust (Vergil), and in the end, he had to finally make his own decision, and that choice was freedom for everyone.
Mundus controlling the world through debt and subterfuge instead of brute force I reply with
my earlier post on how ingenious something like that actually is.
Comparing the endings, I don't think you understand what "cold blood" means, because DMC3 certainly wasn't a battle fought in cold blood, it was reluctance. Similarly, so was DmC's final battle, and Dante feeling lost and alone is an ample emotion to feel considering he's one of two known from an extinct race, and the one person he felt he could really relate to, his brother by family and race, left him.
Dinging a game because "no one asked for this plot" sounds really damn selfish, and it can be creatively robbing. nobody asked for DMC1 when we wanted Resident Evil 4, but hey! People liked it! People didn't ask for SquareSoft teaming up with Disney, but BAM! massively successful new franchise. Regardless of what people ask for, DmC is what we got, and plenty of people liked it.