I thought SG said "He didn't have to go upstairs"Drive-Brys HAHAHA one for the Bryisms Book of Stupidity.
I think BK felt he HAD to do the deed that night. Anyone with any sense, especially a CJ major, would have known the odds of someone being awake with the DD delivery just having happened was more than astronomically high, it was factual.
BK did it anyway for some reason that I can't put my finger on. Was it for the thrill? I have always thought he meant to kill them all. Was his focus on one primary target? It could well have started that way, but the fact that he could have walked down those stairs after killing Maddie and Kaylee and out the door in a straight shot without having to murder Xana and Ethan makes me wonder. He was in disguise, even DM couldn't ID him except for his Brybrows (courtesy of @Megnut).
I can't get it out of my head when Kaylee's dad asked early on "Why did he have to go upstairs?" I'm not 100% convinced of the order of the killings. This fence sitting is killing me.
JMO - today
That means he chose to