I was thinking about the what David Roth has told the Police and I don't think that it is her denying his sexual advances that caused him to strangle and shoot her. I mean he was a 20 year guy. I am pretty sure that a lot more girls rejected him than just her. Believe me, I was 20 once ;-) If that was the case he probably would have killed someone long before then.
There must be something else that triggered him off. I have heard that he said he now thinks that it was because she told him that the two guys she was living with would hurt him. But he choked her first, it would seem like he would shoot her first if he was acting out of fear.
Also, I think she was staying somewhere in Everett, because she didn't have any change of clothes, money. I am wondering if she was vacationing there with the two guys. She probably was hitchhiking to find someone with some booze. She probably didn't know anyone in the area so, she was bored and that would make sense.
It could be possible that the two men that she lived with were involved in some illegal activities like selling drug and didn't report her missing because they didn't want any police attention.
I might check to see if there was anything going on in Everett or Seattle around that time she might have been going to.