Are you referring to when Mark is in jail and meets with Bryon at the end of the novel? When Bryon is finally able to gain access to see Mark, he recognises that his former best friend had undergone a terrifying change. Note how Bryon describes him when he first sees him:
His strangely sinister innocence was gone, and in its place was a more sinister knowledge. He seemed to be pacing, like an impatient, dangerous, caged lion.
What is key to note is the way that Mark has lost his "sinister innocence," only to have it replaced by a more "sinister knowledge." Mark, throughout this excellent novel, is compared to a lion, but now this imagery emphasises the threat and danger that he represents. When he talks to Bryon, Mark says that "I don't seem to be able to get away with things any more." This of course relates back to his loss of "innocence" and its replacement with knowledge. We can perhaps infer therefore that what Mark misses most about his time before he was arrested was the way in which he was able to get away with things because of his innocence. Now, he has entered a time when he has had his innocence replaced with experience, and it seems his character is fundamentally changed forever as a result.