"That she went to the going away party with him that evening instead of going home to Mom leaves me wondering tho."
Maybe he threatened her... and then he still killed her... That's my guess...
as I said on another thread,
she should have said, oh, that's OK honey, I won't embarrass you ... then waited till he was asleep or gone somewhere, gotten outside help so there would be eyewitnesses, confronted him, packed up a few essentials, moved out, and made sure to never be alone with him again.