It's not so much the fact that he killed the dog but the way he killed the dog was deeply disturbing. I find myself FF through that part on rewatches. He fkn hangs the poor dog before finally shooting it through the throat making the poor animal suffer instead of one through the back of the head. I think the scene was meant to show that Michael was indeed not all there. If you're impression of Michael, up to that point, was that he was simply a precocious kid that was just using his smarts to earn some money but overall he was all right, this is how you find out how wrong you were. Fkn kid is sick and you have to be to pull off that caper that gets a bunch of people killed including his own friend.

Great film though. Still holds up. I hadn't watch it in years and found I still liked it. I still don't like the score though. Sounds like some 70s after school special music.