I think it's more the mystery of why he was making his way there in the first place ....sure he may have killed the couple, sure he may be a drug addict....
but how did he get their postcode and why was he going there...
it's the mystery that lures the viewers in and I am starting to think now that the key to all this is Grace
I doubt he killed the couple. I think when he broke in their house their throats were already slashed, and due to fear / loss of self-control he foolishly picked up the knife. Or he didn't even break out, he might have known them and was just visiting (why steal their stuff? Well, they no longer needed them and he was a junkie).
We've been getting quite a few subtle hints about that, with the strongest being Adam's fear when he called his dad from Spain. Adam must have been involved with the wrong kind of people, and these are likely to be the real murderers due to Adam's debts or some other reason.
Now, why was he going to the parents? I honestly have no idea, to tell them he didn't kill them? But that makes little sense, what else would he say? And they would surely call the police, presumed guilty or not. Maybe he knew who killed them or was a witness or something.
Fanboy : a person who does not think while watching.