This is pure speculation, but Wondergirl has me thinking now. Maybe having Lauren appear to be a mysteriously vanished person was just the first part of his plan, such as it was. McD had to realize eventually -- within a couple weeks, at the very most -- it would be assumed Lauren was a murder victim, not just a missing person. So then a closer examination for any clue as to her killer would begin.
A few weeks later on, an investigator looking into Lauren's disappearance would certainly note the coincidence of the maintenance man leaving his job at roughly the same time she went missing. Then maybe they would start inspecting areas the maintenance man had access to, just to see what they could find. Anything remotely suspicious would have been examined -- likely including a hacksaw. If Lauren had been gone with no trace for two or three weeks, and then a hacksaw with Lauren's DNA on it turns up in the shed, things would not have been looking very good at all for that maintenance man.
I had been assuming all along Stephen was acting out of desperation, that the killing happened in some horrible moment of unexpected opportunity, and then he frantically did whatever he could to cover it up and just make it go away. I still sort of think that's the case, mostly. But... maybe not. Maybe it was more malicious and less out of desperation than I wanted to believe.
