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Why do you think Smith ran?

He has a guilty conscience about something...

He had pictures and news article plastered on his wall so he does look guilty to me.

he's had other run ins with the police. also, "James Smith"? really? I assumed it was an alias so I wasn't surprised when he took off.

I agree with you all. He's likely got good reason to run from the police--however I don't think he ran because he's the killer.
I wonder if he knows Laura is dead. Perhaps he saw the killer abduct her.

People are nervous about the police coming to see them even if they're innocent. He may not be innocent but he's made himself look a whole lot more guilty by running. He probably also knew that he'd have to explain all the things on the wall.

His photo collage makes him another automatic suspect.

Somehow it seems to me as though the real killer left pointers for the police to stumble onto James Smith. They never got to see his face. Though is surely a stalker, and is surely not called James Smith, I'm not sure he is our man.