John, I just read your post again and am adding a bit more to my original response. I'm sure you could argue that Mattie was not acting in vengeance the second time she shot Chaney. I think her previous experience told her that he was not going to go with her willingly. The situation was more dire now; people are dying all around her, and nothing had gone according to plan. She may have realized that the only way Tom Chaney was going anywhere with her was draped over a horse. Certainly she had her personal vendetta against the man; it's the reason she was even out there in the first place. But her lack of experience in general combined with her harrowing experience in the present excuse her a bit more than if, say, it had been Rooster doing the shooting. I think that you're right that there were consequences to the adventure as a whole; I think it's why she never married (although having only one arm in the Old West would probably make a woman much less desirable) and that it affected her entire demeanor. That's a life-changing experience for sure for a little girl.
I'm glad you've enjoyed the film enough to watch it three times, but I fear that you will have the remake so much in your head that you won't enjoy the original. I think Adrienne is correct that the original needs to be seen first. I would wait longer in between viewings than I did, though, because I kept filling the gaps of the remake in with details from the original. And because they were so similar, it's hard to keep my opinions of the interpretations separate. I will say that more than many other films, I am interested in reading the book as sort of a final word on the matter. It really is a remarkable story.