All the following is MOO.
I feel like Kaylee was the sole target. BK entered through the patio into the kitchen and immediately went upstairs. I don't think he expected Maddie to be in the same bed. Unfortunately he felt he had to kill her too.
I think he was going to leave after that but then ran into Xana eating her DD. She might have gotten up from where she was hanging out in the living room to see who was walking around late at night. Or she might have been eating in bed with Ethan either sleeping next to her, or he could have been awake too ("Someone's here"). If she was in her bedroom (and didn't run there if she had been in the LR) perhaps she opened her door like DM did to see who was out there. I do not feel either Ethan or Xana would have been killed if they had been sleeping behind a closed door, or awake but didn't open the door to look out. To me, BK was focused on Kaylee and then getting out of the house ASAP. Unfortunately his plan went awry when he found Maddie in bed with Kaylee.
However it was that he ran across Xana (either in the LR or opening her door to look out) that was when it was decided she had to go too. Then Ethan. Basically, he was getting rid of all of the witnesses.
Then why did he not kill DM? Great question. Perhaps his mind was on overload at that point since killing 1 turned into killing 4, and he was focused on getting the heck out of there with the quickness. Maybe he didn't see her as he could have been blinded by his thoughts of what the heck just went down in the last few minutes. Especially if he had only planned on killing 1/Kaylee. He had to be pumped up on adrenaline too. It might have seemed like his fast beating heart was deafening (I actually once heard my heart beating so loud I thought everyone close could hear it. Not sure what that was about, but due to that experience I know it's possible to have your ears filled with the sound of your heartbeat. It's a tad disorienting and all my focus was on it. Maybe the same for BK.).
Lastly, one of the other reasons I feel like he only planned to kill 1 was because he peeled out of there laying rubber at ~4:20am. Stupid thing to do if you're trying to sneak off after murdering someone! You for darn sure don't want to bring attention to it or yourself until you're far away.
I feel like had Kaylee been alone in her bed, it would have been just her he murdered, then quickly leave the house, and then drive off in a more controlled manner (quick, but not crazy fast). It just feels like he freaked out because his plan ended up killing X4, and quite frankly, I feel like he spooked himself and got the heck out of dodge. FAST. Also, maybe he did see DM but wanted out of that house ASAP and knew it was only a few more yards to 'safety'. Seeing her could also be the reason he peeled out so fast. I think she got really lucky that awful night.
All MOO.