At the end of the book That Was Then, This Is Now, what does Bryon realize about Mark?
At the end of the story, Bryon realizes that Mark is unable to tell right from wrong.
Bryon has just returned from the hospital, where his girlfriend Cathy's brother M&M has been admitted while on a bad, extended LSD trip. He is consumed with worry for Cathy, and grief at what has happened to M&M, who is still just a kid. At this point, he discovers, under Mark's mattress, proof that Mark is dealing drugs. In Bryon's thinking at moment, "M&M had lost his mind and Cathy was hurting, and he (had to do) something about it". Bryon calls the police and turns Mark in.
Before the police arrive, Mark comes home. Numb, Bryon explains what has happened to M&M and shows Mark the drugs he has found under his mattress. Mark is nonchalant, says that he is selling drugs because they need the money, and insists that it is all right because he himself is not taking the drugs. Mark does not see any problem with what he has been doing, and Bryon realizes that
"Mark had absolutely no concept of what was right and what was wrong; he didn't obey any laws, because he couldn't see that there were any. Laws, right and wrong, they didn't matter to Mark, because they were just words".
Just before the police arrive, Mark suddenly notices that Bryon is upset and agitated. Misunderstanding the gravity of the situation, he tells Bryon that "if it bugs (him) that much, (he'll) quit" dealing. Even at this point, Mark cannnot comprehend the inherent wrongness of his actions, and he is aghast when Bryon tells him that he has already "called the cops" (Chapter 10).