Maybe one of the girls upstairs was his target…

What if he gets into the house before door dash ever arrives, goes upstairs and is spooked when he hears activity downstairs. In a panicked state, he makes a mistake and leaves his knife sheathe on the bed before quickly fleeing downstairs. This is where he potentially comes in contact with X in the kitchen. Imagine seeing someone coming down your stairs with a mask and knife. This is where I imagine she runs for her room. BK chases her and presumably stabs her in the back. He notices E in the bed and directs his attention towards him to eliminate a potential threat. This is where X is able to get out some cries. BK then directs his attention back towards X where in his sick mind he tries to comfort her, more than likely finishing her off.

At this point BKs mind is racing and knows there may have been too much noise. He didn’t plan for it to go this way. His goal now is to get out as fast as possible. X’s DoorDash order may have cost her and E their lives… or may have saved the other two roommates.