We also aren't privy to his plan or intent.
It was a dynamic situation for which he may not have adequately prepared.
K being there and in M's bed.
X being awake for doordash.
E being there, awake or awakened.
D being awake, calling out.
At 4 am, BK may have anticipated a static situation. Everyone asleep, one sleeper per room.
It's possible he had one target, one and only one.
It's possible he planned to enter, kill (just one), exit in mere minutes.
He might have accomplished that in virtual silence, if not for the dynamic features bulleted above.
It's possible that he intended to spend more time there, with just his one victim. Controlling with the knife. Taunt, torture, SA. If his plan was one decisive murder, without the others, the sheath might never have left his second hand.
It is likely his plan, whatever it was, was impacted, if not thwarted or accelerated, by the dynamic features. Encountering K would have resulted, as it did, in noise. And noise is what awakened D. Assuming he heard her utterance, he now knew he had a complication.
When he descended the staircase from the third level, he likely encountered E. IMO it's possible that the bedroom light was on. (If BK turned that light off, after murdering E and X, as he left that bedroom, the adjustment from light to dark may have affected his vision enough to miss seeing D, her door or the movement of it opening. Maybe?) If BK's target was just one, then IMO K was a surprise to him, as were E and X. He may have ascribed D's utterance to X because he saw that she was awake, may have been unaware that D was there at all, therefore.
In a matter of minutes, his careful plan to murder one victim escalated into four victims (if you accept the premise of a single, targeted victim).
Additionally, it is possible he was aware of Dylan. He may have made a calculated decision. We don't know his mindset. Not getting caught may not have been as compelling to him as the satisfaction of what he'd accomplished, assuming sick gratification as his goal. Perhaps he made a calculation. We know D locked her door. He may have heard that, did the math, decided the effort and efficacy of breaching her door was beyond his scope and opted for escape.
Knowing he left a live witness may have increased the thrill -- certainly ups the risk of being caught. Explains the rapid exodus, might explain his circuitous route home. He expected LE to come in hot.
But he knew he'd just massacred four people. D did not. She only knew that she'd been awakened by her roommates, did not register fear but annoyance, opened her door the third time, to be shocked to find a man directly outside her door. But even so, he paid her no mind and continued toward the door. Easy to read walk of shame not denouement of mass murder. She went back to bed, probably comforted by the silence which is what she sought in the first place. Eerie silence came later IMO.
My point to all of this is that, had BK encountered only M in her bed, if she was his target, and no one else had been awake, he may have entered in near silence and left the same way, leaving one murdered victim. Swift and efficient. Scary as hell.
Even with the moving parts listed above, he was in and out in under ten minutes, plus or minus. Leaving a swath of human destruction. Scary as hell times four.
He is the monster under the bed.
JMO