Hmm, i thought it was the other way round (bottom door open, top door with key in) but could be wrong.
Either way i guess it's possible he was deciding as he crept up the stairs (rail is LOW, so he was crawling/creeping to be using that instead of the banister) what he was going to do. I think he went looking for drugs, and i think he took the knife to protect himself from Rab, not to threaten Alesha. I think he probably just came across her quite near the door, given the squeaky door and many people in there sleeping who didn't wake. I read earlier someone's theory that the angles of the CCTV from the shore made it look like he was carrying something on his back in one image and something dangling in front of him in the other, the inference being that she was actually over his shoulder in both cases. Like a fireman's lift. So unconscious?