Fascinating, yet horrible, case. I'm thoroughly impressed with some of the members' devotion to this story, and to exploring all possible (and some seemingly impossible) angles. Sam clearly had a horrible home life, with no familial support, affection or love of any kind, at least in the recent past. I believe part of the reason he hung around the house for so many days after the murders is because he was enjoying having the place and space to himself. He wasn't holed up in his bedroom, avoiding the insults of his sister or the depressing scene of his mother smoking pot and watching TV all day. I believe he was quite content eating the Kellys' food, playing on the computer, watching TV, going grocery shopping, and "playing house" for a bit of time, as sick as that may sound considering there were three and later four bodies sharing the space.
As far as why he took Mark's car, I don't think we can ever know the answer to this unless Sam reveals it. For all we know, he may have just needed some air (no pun intended, ahem) and a change of scenery and gone out for a joyride. Using the car for disposal of evidence or scouting for a possible body dumping site doesn't sit right with me - I think that's giving him too much credit and foresight.
And, finally, it seems we've determined, due to Peace Gurl's valuable, much appreciated information, that he called the skinny policeman to the house, in particular requesting to the dispatcher the 'same one who came before', in order to probably murder him as well. Perhaps by this point he was 'coming down' off the high of killing the other four victims and was craving that adrenaline rush and feeling of power again.
