I'm by no stretch of the imagination calling them innocent. But to call them totally evil, as opposition to the question of the Dawnguard being too good, is going a bit far. They do actively attack, yes, but they do just as much as a defensive measure against attacks and their way of life.
It may be amusing, but it is still a lifestyle. Everybody tries to defend the way they live. That bear of yours may be a problem, but it will retaliate if attacks. It does have a den, it will protect its young, and will act within reason to protect them. Who's the say the vampires aren't doing something similar here? And I'm not saying the dawnguard or humans acted first, but there is definitely a cycle going on no matter who acted first.