This film is a perfect example of the recent crop of horror films that simply are not fully realized. There are two routes to take in horror films: either you don't really explain what's going on (or who the killer is, like in Texas Chainsaw Massacre) or give the characters a lot of back story and characterization so that everything is explained (Halloween could potentially be an example of this).
Unfortunately, Creep fails in this area. I see absolutely no reason to give a small shred of the back story for Craig without fully capturing the essence of his character or his motivation. No character is fully realized, although the circular nature of Kate's character in the film is the most accomplished part of the story.
In the end, this story is mostly unmotivated and none of the performances give it the necessary life to make it enthralling or interesting enough to overcome the lack of context and empty film-making that drag down the film.
This film will go down as another example of a film (as most are) that could be so much better even though it would never be great. The only thing that was used properly in the film was the setting: the characters, story, direction, and overall writing would have benefited from a serious face lift.
Creep does serve a purpose as a mindless, silly horror movie with no intellectual or emotional investment, but sadly, that's about it...