I tend to agree that this killer had not planned for more than one murder that night.There would just be no reason for him to use something like that, and it would leave traces as well. It would draw attention, and give him away.
I'm convinced this was meant to be a single homicide, and he intended to sneak in and out undetected by the rest of the household.
I think Dylan was just really drunk, and she imagined this. There's nothing plausible that makes any sense to me other than that.
And, it is possible that Dylan might have been hearing something but was in and out of the reality realm. It could be that some of what she reported was what was happening and the rest was her brain trying to make sense of it through the fog.
Regardless, it is a heavy burden to bear to be a survivor in that horrible crime and feel that you might have been able to save your friends. She is shattered and will never be able to have a sense of peace without serious help and support for many years.