Well in my opinion, it's like the civil war you choose the lesser of two evils. Sure dawnguard want to kill people who have no choice but to feed on people but at the same time, the vampires want to block out the sun. So again it's the lesser of two evils
Actually a good portion don't care, they are just following Harkon's orders.
Not really, certain books can be contradicted by other books or characters. Serana and Harkon mention only Molag Bal as the creator of vampires and violator of women. Since serana, Harkon and velarica have a first hand account of what molag bal does, I believe them more than a book written by someone could have had limited knowledge and lets not forget that the denizens of morrowind stupidly believe some actual evil daedric princes to be good such as boethiah or whatever name was.
Dawnguard like other vampire hunters are a bit overzealous but think about this...what is currently happening? vampire attacks, vigil of stendarr slaughtered and there are plans to blot out the sun etc. The dawnguard reformed for a reason.
Vampires are not really evil though, it depends on the person and not what they are but some do go feral and wild or let the power go their head.
Daedra are neither good nor bad, the concept of evil and good doesn't apply to them since its a mortal concept and they live in a higher plane of existence, also Boethiah isn't really evil if it weren't for him the Dunmer wouldn't have the house system and ebony wouldn't have been made into armor or weapon thus not giving Morrowind its financial power in trading minerals like ebony and glass.
Now back to the real question, I'd say yes and no. The vampires are just trying to survive and most really have no choice in the matter of being a vampire but the problem is there are vampires who embrace the gift too much and go mad with bloodlust and will attack anything they see or enthrall them while the Dawnguard just want to protect their own races but I think that persecuting all Vampires unless they help the dawnguard and later cure themselves is kind of stupid.