Dude, what the hell!
Alright, so before I rolled my latest character, I decided to purchase Dawnguard, because the new weapon and faction options seemed really awesome. So, I take care of that, load into my new game and begin. Seven hours later, I'm doing my thing in Whiterun, and all of a sudden I see people with weapons out near the front gates going buck-wild on each other. It turns out, a Master Vampire* straight-up ran into Whiterun, said, "whattup guise," and murdered Jon Battle-Born and Sigurd! I had to interfere with my warhammer and crush his skull, considering he murdered two people.
How is this going to mess up my new game if these dudes are going to roll into town and kill actual citizens, and probably even a quest giver at some point? How many others experienced this, and how prevalent is it? And to clarify, I have not completed any missions in the Dawnguard questline.
Fucking vampires. Christ.
EDIT 1: Now, I'm trying to figure out if I should reload my save to before this occurred, preventing possible quest issues, or just roll with it and accept it as a part of my game, any [possible] quest issues be damned.