Im gonna speculate that the perp went into the house & up to the 3rd to commit murder first....my reasoning is he first pulled out his knife probably standing over Maddie, & laid down the sheath next to her before committing the horrific crime, & as he was doing this, it woke up Kaylee, hence roommate B on the 2nd floor saying she thought she heard Kaylee playing with her dog.
The dog was in another room & must have been barking once the perp started attacking Kaylee, & im only assuming here, but Kaylee might have defense wounds on her trying to fight off the perp, & that might explain why the perp was more vicious with her as her father states she had more stab wounds than the other victims.
In the perps frenzy, & having to move quickly over to Kaylee because she woke up hearing what he was doing to Maddie, he forgets all about the knife sheath as his adrenaline is pumping & decides to go down stairs to the 2nd floor.....the thing is, roommate B was also in one of the rooms on the 2nd floor, but im assuming with his 11 or 12 times surveilling the girls, he knew exactly where their rooms were, & again im assuming that Maddie & Xana were the targets, I could be way off & wrong in my assumption, but thats how I see it as of now.
As he enters Xanas room, he probably takes out the biggest threat in the room which is Ethan, & thats when Xana wakes up & probably cant believe whats shes witnessing, shes probably attacked fairly quickly & as shes crying, her roommate B heres the perp stating, its ok, ill help you.
Few moments later, roommate B opens her door & she sees the perp walking out of Xana room & towards her then pivots & out the sliding glass door.
The perp forgetting the sheath next to Maddies body & him leaving out of Xanas room before leaving the house tells us he went to the 3rd floor first imo.......
I also think these murders were his first because of the many mistakes he made. Good thing they finally caught him because he might have enjoyed the taste of blood & murder too much, he might have tried it again in the future.