Correct me if I'm wrong, but one does not "catch" humanity, but one does "catch" vampirism. So a cured Serana is a diseased Serana...how? She's already diseased. I dont cure her for the sole reason of the fact that the only thing that changes is her eye color. And people always say that evil is a point of view. Well, from a point of view, vampires are evil. If I said the same thing about Adolf Hitler, that he's not evil, because evil is merely a point of view, heads would roll.
A cured Serana is a diseased Serana because she is weaker, she is supceptible to diseases, dragged down by the weakness of the body. Frail and weak compared to the powers granted by being a vampire such as enhanced senses, immortality by unaging, alterior form, etc. I didn't cure her because she doesn't deserve to become food like the other flesh bags of the world. She's already a vampire, why reduce her prey? Why take her away from her vampiric culture and society which she grew up in? Yeah, it was horrible, but after Harkon's demise and the return of her mother, things will surely change.
If you believe vampires are evil, then that is fine. Because from my point of view, they aren't. I won't comment about Hitler because I will not derail the subject

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I doubt that she regards it as a 'disease', seeing as she willingly subscribed. Also, vampires are not always evil in TES (the majority sure, but you have a few good ones), just look at Hassildor who rules successfully for decades without giving in to his deeper urges (he obviously still fed though, probably from willing members of the court). Both the Dawnguard and the Vigilant are bigoted idiots who thankfully will have killed many evil ones before they find and murder the good ones (Isran would probably kill Serana given the chance, they only don't because they think it would 'annoy' you).
Only completely irrational people would disagree if you said Hitler was only evil from our perspective (I think he is before any accusations fly), from the perspective of those in his party; they were doing the right thing.
Janus Hassildor indeed fed on people, since he embraced the changes within him. But since Clavicus Vile supressed the thirst of his bloodline, then he doesn't require an ocean of blood as other vampires do. They are more civilized in that regard, and even the Pale Lady in his castle was tasked to harvest blood for Rona Hassildor based on her dialogue.
The Vigilantes are hypocrites, Isran is a fanatical extremist and they're all food, so I suppose they deserve to die. Tamriel isn't called the Arena for nothing, right? I agree with everything you said, most of it, anyway.