Having been reading over
hbpspork (I'm hopelessly behind on everything), I noticed how many people were disgusted and angered by Harry's behavior in the book. I noticed it as well: Harry uses cruel spells as jokes when he cannot be discovered or traced to the crime and attempts to use Unforgivable Curses. He shows very little remorse for nearly killing Draco, and ignores many of his friends.
We're being introduced to another character who is very like this, simultaneously, throughout the book, and I have difficulty believing this juxtaposition isn't at least unconsciously intention. Remember how Dumbledore insisted it was no longer necessary for Harry to receive Occlumency lessons? Surely he meant that Voldemort had given up on trying to find, manipulate or just plain drive Harry insane? Surely it meant the bond between them had weakened?
Absolutely not. If Harry's sudden and violent personality change from where he was in OotP is any marker, the bond between Harry and LV is stronger. Don't you think it's possible that Dumbledore wants that link wide open while he explores Tom Riddle's past? Don't you think it's possible that he gets concepts and flashes of insight on the Horcruxes, or on Riddle's feelings about these matters, through Harry? That's why he needs Harry to go over rather pointless and unnecessary bits of memory. Harry's a Polish Mine Detector, poor kid. No wonder he thinks hexing Squibs and risking permanent damage to his friends is funny.