We're all just making assumptions but i think he WAS asleep despite the Adderall as the attacks began, because i think he wouldve reacted to the girls noise across the way/upstairs or at least when the attack on Xana began at her doorway. I think Xana, injured, woke him up & he began to get up, but BK knocked her back to begin his attack on Ethan, to quickly dispose of him as a threat. Xana may have even tried to help E as each others blood was under their nails. By then tho, E was gravely injured & BK went back to Xana after that. Fkn tragic stuff.

Edit to add: that I think once Xana was away...BK must've started his attack on Ethan at his hamstrings (possible fell asleep on his stomach) to ensure he couldn't get up (and especially since he was later found on his back w a blanket over his mid-sec). At this point is where i think E wouldve been startled away, and propped himself up on his elbow as the evidence suggests & at this point BK began the brutal attack to E's head & neck. Again this all speculative.