Is justice served at the end of Monster? What is meant by justice?

In context, this is hard to say. Throughout the novel, it is never confirmed that Steve did or did not play role in the crime. In my opinion, Steve was likely innocent, but those are my perceptions and many would probably disagree. Either way, Steve's difficulty in returning to a normal life speaks to both justice and injustice... his feelings of guilt and inability to cope would be seen as a kind of justice had he actually commited the crime... we might also turn that around and note that if Steve was actually innocent... the cold treatment he received from his lawyer and others after his acquittal might also be seen as injustice.