Yeah the attacks seem kinda ridiculous (on my game atleast, ive heard its pretty random). I'm only lvl 8 and 3 npcs in whiterun are already dead. I cant even start the dawnguard quest line until lvl 10. The attacks really shouldnt start to happen until you start the questline. Edit* I just found two more dead just outside whiterun, one was the stable master. Guess I'm not buying any horses from whiterun now. *