If Derrick believes that line "he" delivers to the MC, about "it's not like I'm selling crack and getting them addicted", then he's an idiot. And if the author wrote him that way, but doesn't believe it themselves, good for them. Genuinely, good for them because they wrote a more believably stupid character in Derrick than they did in Geoffrey (who's more of a stereotypical stupid). If the author does believe that line themselves...well, at least they still managed to write a believably stupid character.