A theory or opinion isn't a fact, like they were calling for help or had info about the soon to be killer...... then, strangely, turned the lights out and went to sleep.
It's not impossible your assumption is correct but if we go with probabilities, I lean towards K's usual habit of late night persistent calls, as her sister described.
They couldn't save themselves because they didn't know they were going to be attacked, that's why this case is so disturbing, it was a cowardly act against defenseless sleeping people who had no idea.
It's everyone's worst nightmare, can't run, hide, fight or protect yourself!
Jmo