It's possible she thought that if she didn't leave when he asked he could have killed the rest of the family, isn't it?
His statement to police was that he couldn't find her after the murders and was driving off when Kara came running down the road and jumped in his car and announced she wanted to get as far away as possible, get married and "start a new life."
I don't know about you, but I never had the nerve to stay out all night with a boy when I was growing up; much less when I was 14.