So you are accepting the fact that they torture vampires for information? I suppose that is not malevolent right? In any case, everything is malevolent.
Lions favor the weakest animal in the group of prey, either the young or the sick, to devour and feast upon. I guess they are malevolent too, right? Especially those two lions known as the Tsvao maneaters notoriously known for taking many lives(possibly over a hundred) just for the hell of it. In any case, vampires naturally kill people to satisfy their thirst and hunger for blood. Valerica herself sees the Dawnguard as the bad guys. And the Dawnguard see vampires as evil. In the end, it is all a point of view. And from my point of view, the Dawnguard are evil(No, I am not lost).
I accept that they are most likely doing it, and I accept from a vampire point of view their are a malevolent group. However, looking at the greater picture I see the Dawnguard as being in the right and at least their goal is to protect innocents and/or avenge people killed by vampires. The dawnguard are not Paragons of Virtue, but they sure aren't the bad guys in this situation, Lord Harkon is.
As for Valerica thinking the Dawnguard being the bad guys: What do you expect? She is a vampire and therefor a possible victim of their cause. Serana is probably only alive on favor of the player and by playing an important role in stopping the prophecy. The Dawnguard are extremely anti-vampire since they(at leasst I assume they do) see every vampire as a potential threat.