DM was awakened at 4 am by sounds, after having been awake and chatting with roommates just hours before. At the max, there was noise from 4:10 to 4:20, but more likely, 4:11 to 4:17. Six minutes. Quiet before. Quiet after. Sounds that made no sense, state of alert leaving her unsure if it was altogether real or not.
He may have been upstairs for less than a minute, his initial attack complicated by the presence of KG, which awakened DM, which led to BK racing down the stairs to silence DM, but encountering XK instead, it's okay, I'm here to help you. Probably said that near her doorway where he realized he had yet another threat to contend with, EC.
BK spent 5 minutes in XK's room, by my maths.
4:00 XK gets her door dash food, shuts the slider
4:11 BK enters, leaves slider partially open
4:11 XK is in her room, on TikTok with her food
4:11 BK murders MM, lets go of the sheath, bludgeons KG
4:11 DM hears sounds upstairs, which she thinks must be LG and Murphy. I believe she yelled something to her roommates which BK also would have heard. XK might not have heard anything from upstairs or from DM because of her airport.
4:12 XK goes to the kitchen, sees the slider open, says there's someone here, DM hears her (DM thought KG said it but DM assigned all the sounds to KG, she didn't know XK was awake)
4:12 DM hears KG on the stairs. But it was actually BK going up the stairs at 4:11 and quickly back down, in under a minute IMO
4:12 BK sees XK returning to her room. She's a threat to his escape, can't let her call 911. He reaches her, perhaps from behind, quiets her whimper by saying calmly, it's okay, I'm here to help you
4:12 DM looks out her for, sees no one
4:12 XK drops her phone, no more activity on her phone from her
4:13 BK attacks XK. XK tries to stop the knife, is injured badly. BK steps over/around her to kill EC, XK cries, BK returns to her and mortally wounds her
4:14 I think BK may have sat for a moment, to gather his thoughts. Among many miscalculation that night, he may have felt like he'd put out three additional pop up fires and that there would be no more. From this vantage, he may have returned to EC to inflict gratuitous injury. The ME will know if the carving was pre or post mortem. Why do it? Sister's movie. In that movie, the victims were all strangled and each had initials carved in their chests, by a very large knife. This may have been BK's underlying script. From the moment he saw that movie, he may have fantasized about it. Based on his age at the time, it might have cross wired as a sexual fantasy which, into adulthood, he paired with the very SKs he studied. In other words, he always knew he'd do it (kill a girl as SA), and he always wondered how he would decide whom to kill. Hence, his Master's intetest and questionnaire.
4:16-:18

M opens her door, sees BK
4:17 garbled sounds, THUD, barking dog.
4:19 DM starts calling/texting roommates
4:21 BK speeds away
JMO