Destroying something that took years of work is a terrible, terrible thing. People--especially kids--sometimes are so overcome by emotion (even petty emotion) that they act with no sense of proportion or consequence.

Exactly! Amy doesn't burn Jo's book because she is the Bad Sister and everything she does is Bad, she burns the book because she's angry at Jo, she knows it will hurt her, and she has no understanding of what actually goes into the writing of a novel, even a terrible juvenilia novel, so she's done more harm than she even intended. It doesn't point to a future of wanton destruction. It points to being twelve.