When Jolie returns to Gena Rowland's house, and goes to the basement, who was that guy under the bed who scared her? The killer (I don't mention the actor, because someone may have not seen the movie)? What was he doing there?
It was a very fast, choppy segment. To me, the guy's physique looked too large to be Martin, and he appeared to be wearing old, shabby clothing. Shortly after that scene, the police tell Jolie that the person was someone (a transient or a homeless person?) who had broken into the house for food and shelter. I'm not sure if they just surmised that, or if they had actually captured the person. The guy ran away pretty quickly after Jolie kicked his leg and knocked him over. If it was Martin, I don't think he would have fled so easily.
I don't think it was Martin either. He would have said something when he was revealed, or the director would show us a flashback or something. And like you say, this guy was big. I think it was an unnecessary scene. A cliche "boo!" moment.
I think it was Martin, who possibly wanted to kill his mother, or just thought it was funny he could be there when the cops were looking for him, or maybe he wanted to go back to his old place where he hatched plans of killing people.
It had to be Martin revisiting his earlier haunts. But I agree with Ben above, the guy's physique looked too large to actually be Martin, so in that respect I think it was very much a red herring.