"He didn't have to go upstairs."
If X was his target, E might've been a surprise. X being awake might've been a surprise. Two dead.
He wouldn't have needed to go upstairs.
If M was his target, he needed to go upstairs. K, collateral.
If M was the target, why kill X? Why kill E? Why leave D?
IMO the answer is timing.
He had to go upstairs to reach his target.
He opted to come back downstairs -- to leave.
IMO if X and E were behind closed doors, like D, if K had been in her own room, I think his target would be clear. One dead.
Swift, silent, every bit as stupid (vehicle caught on CCTV, phone usage up to and after) and still altogether evil. Might've left with his sheath however.
He didn't have to go upstairs. But he did. Therefore, IMO, his target was on the third floor.
So relieved he's been arrested.
Praying he gets the fairest of trials and the fairest of convictions, so he never walks among us again.
JMO