Yea a few people have said that the dawnguard after awhile start looking like complete a-holes, honestly I can see them as acting like an Inquisition like group doing whatever they can to hunt down vampires, even if it means roughing up or killing a few civilians.
That's kind of why I joined the vampires and not the Dawnguard for my first play through to be honest haha. I'm wary of anything fanatical or too strongly religious based, and my first run in with the Dawnguard rubbed me wrong. They were 100% fanatically obsessed with killing vampires (not exactly bad not exactly good) but the way in which (spoiler... Even if it's just of 5 minutes in) they treated a Vigilant of Stendarr just wishing to gain aid was just cold to me. They felt nothing like good guys. Then after meeting Serana and seeing how charming she is, I chose to join up with the Vampires and stick close to Serana, keeping her safe.
Not to mention the use of armored trolls by the Dawnguard also rubbed me wrong... These beasts are destructive, dangerous, and difficult to control, yet the Dawnguard equips them with armor and blades on their armor. Sounds dangerous and desperate haha.
TL;DR Dawnguard felt like bad guys, Serana specifically felt like the only good gal there.