In the book "Monster", did Steve change and if so, what was it and was it good or a bad change?

The primary change in Monster is how Steve views himself.  Again, we really do not know if he is innocent or guilty.  Perhaps, it is the author's strength in description that leaves us in the middle of trying to determine for ourselves what happened the night in question.  However, we do know that Steve has changed.  His change has resulted from the trial and the emotions trapped within it.  He has to balance what he knows he did in his own mind, with how the prosecution has depicted him to the jury, how his own lawyer depicts him, and how she views him on a personal level, and how his family, especially his father, will perceive him.  The most important change Steve undergoes in all of this is that his perception of self is a combination of all of these view points.  In the modern setting, there is no "one aspect of universal truth."  Rather, truth is a collection of figments and fragments, in the hopes of understanding some aspect of a whole. The author knows this and plays this to an enhanced degree when Steve has to understand his own sense of self.  The style of the novel contributes to this, as it is in script and journal entry form.  There is no "complete" and "universal" truth in narration.  Just like us, Steve's change is that he is now seeing himself in multiple and different lights, some that show him not to be a monster and some that do.  His change is the confused and diverse understanding of self he now possesses.