In the quest DLC1_WESC05 (Dawnguard Vampire Hunters), the Dawnguard NPCs are way too aggressive. This is obviously intentional as far as the player is concerned, but they seem to attack and kill basically everything that moves.
Here are some examples:
Guards (other non-hostile NPCs) - They attacked and killed the guards at the Western Watchtower after spawning nearby. Also, the encounter spawned near the main gate outside of Riften and they killed the guard there as well as attacking the Khajiit caravan camped nearby.
Farm animals - All of the farm animals and the two dogs at Barleydark Farm (added by CRF) were killed when the three Dawnguard spawned nearby. This also occurred at Snow-Shod farm where they not only killed the livestock, but also killed the farmer and his wife there.
Wild animals - When "non-hostile" wild animals like deer, elk, foxes, or rabbits are nearby the NPCs go out of their to kill them. This would be expected with aggressive animals like bears and wolves.
This may be due to a setting in one or more of the actor templates used in the WE quest. Not sure why, but I haven't noticed the same problem with the Dawnguard vampire hunters in a similar encounter - DLC1WE08.