Who says it has to be a serial killer?? Could be some guy who she rejected, and he lost his temper the one time and lured her out of the house somehow. Not every murderer turns into a serial killer. Some of them only do it one time. Of course it COULD be the hubby, but the facts that you all have pointed out do not exclude other people.
The one fact that keeps bothering all of us is, her leaving the house in the wee hours of the morning. BUT... stranger things have happened. Several years ago there was a young woman in my area that was abducted from a Wal Mart parking lot when she was getting off work, (her car was found in the lot the next day) and the guy assaulted and killed her, then left her body off the side of the highway, I think in OK, if I'm not mistaken.
IDK... I just hate to be stuck on one suspect, just because... it's almost too easy. Think outside the box... that's my motto.