Its not a bug. Hroggar is under Alva's spell. If you want to kill him, go ahead, you won't get into trouble. But if you plan on becoming the thane of Morthal, you need him for a later quest. In order to bypass Hroggar without killing him, you need to go to Alva's house during the day when Hroggar is working at the mill. Go to the basement and kill Alva, Hroggar lives, problem solved. That's what I did. But it isnt necessary to kill Alva. When I attacked her, I used the wobbojack staff and when I hit her with it, she ran out the house, leaving the journal behind. All you really need is the journal. When I took it and went out the house, Alva was acting normal, but running. I attacked her again, but the guard and towns people started attacking me.