Steve is questioning his identity in a couple of ways. The most prevalent way is in his wonderfing if he actually is the "monster" that the prosecution makes him appear to be. Steve does not have the strong moral grounding that would enable him to reject such a characterization. If he did, there is a good chance he would not have made such bad decisions leading to the crime.
Steve legitimately begins to wonder if the way the prosecution is depicting him is really the way he is: "It was me who lay on the cot wondering if I was fooling myself." In the end, this becomes how Steve is questioning his own identity. As reflected in his screenplay, he truly sees himself in one particular light. Scared of the realities in prison, and wishing to reclaim his freedom, Steve wants out. However, Steve recognizes that within his fear and uncertainty of what lies ahead, there is the existential reality that he is questioning who he is. The combination of both doubt and fear help to make Steve reflect about his identity.