They didn’t know there was something they needed to wake up and deal with.
RSBM
I don't agree, in this specific case, the facts are very clear, I'll outline them below.
IMO the roomate who saw/heard the crime was in a literal trauma response (not a pop psychology one). Just as, if you're in a serious accident, your mind will not allow you to think about what happened or what you should do. You will just be focussed on finding safety, like a deer hiding from a lion.
This happens so often, people critize a victim for not acting rationally when they are injured or lost, etc. But their problem is, their rational mind has literally turned off. The organism doesn't want the person to do anything stupid.
In the released policecam footage, the roomate who saw BK said she was utterly terrified that night and had to work up her courage to dash downstairs and hide/lock herself in with the downstairs roommate, who was the only one who answered her repeated texts to all the girls in the house.
The next morning the two survivors kept texting the others and were too afraid to go upstairs, finally contacting a friend, who brought a guy over, who was the only one willing to go upstairs, although first he went and got a sharp knife from the kitchen!
IMO, that's because although the witness couldn't say what she saw, she emotionally conveyed her genuine terror.
So it was not the situation that they didn't know something terrible must have happened. Otherwise, why didn't they all go up to Xana, who the guy had glimpsed lying on the floor, and check how she was. Instead, they left the house and huddled, dressed in their pjs, out in their parking lot in near freezing weather. And told 911 their friend appeared to have passed out... But if she'd only passed out or had an OD, why were they out by the garbage bin?
Terror, that's why.
JMO