I really wonder if DM was under the influence of something, and questioning what she thought she saw (or just assuming it was much less dire than we now know, e.g. a burglar who left). If she was at all sure of what she witnessed, I'd think she would call the police (or at least several male friends) out of terror for her own self if nothing else. I really doubt this was just out of lack of concern for her roommates or something like some are implying. Not only did she not call, she stayed in the home all night. If I really thought I'd just seen a literal murderer in my home, I'd be out of there like a bat out of hell screaming my head off. It's not like they're in a rural area in the middle of nowhere with no neighbors to run to.