How do Mrs Dubose's camellias symbolise injustice?
In my opinion, the camellias themselves symbolize innocence more than anything. However, I think that you can say that what Jem does to the camellias symbolizes injustice. One of the major themes in this book is that people do unjust things because they do not really understand (or care to understand) other people. The whites persecute black people like Tom Robinson. "Normal" people persecute "weird" people like Boo Radley. This is unjust because the people being persecuted don't deserve it, but the persecutors don't bother to "walk in their shoes" and understand their victims.
This is what Jem does to Mrs. Dubose when he kills her camellias. Sure, she kind of deserves it for how nasty she is. But Jem does not know the whole story. He doesn't really know who she is and what strains she is under. When he kills her camellias, he is lashing out at his image of her -- not at who she really is. This is just like how others in the book lash out at their images of bad black men or of strange "monsters" without being just -- without taking the time to truly understand the situation and treat others in a just manner.