Why did he not kill her? I guess we will never know.
I think he heard DM holler up to the third floor while he was on the third floor. Then as he descended the steps, he must have encountered XK, perhaps as she brought her DD to the kitchen. If she was listening to TikTok, she likely had earbuds in and didn't hear any disturbances, including DM's utterances... or IMO, she would have responded to DM herself.
When BK came upon XK, he likely assumed she was the speaker he heard, and after the slaughter in XK's room, he would have been hellbent on getting the hell out of there.
I don't think he realized he'd seen DM, until he was past her door. I think he may have even come back to her door, listened at it, shrugged it off as HIS mind playing tricks on him, and then left, just as DM was calling KG.
BK had maybe two minutes to strip off his clothes or cover them up, stash them in his car or trunk and squeal out of there.
Why speed away? I think it confirms that he realized he did see someone and knew that someone had seen him and would already be on the phone with 911.
911 at 4 in the morning, to a scene with four bloody murders, what do you suppose the response time would be? Minutes IMO. BK knew he had to get out of there and fast --
JMO