"... I was really hoping you'd grow out of this obsession with murderers."(By "we", the speaker means normal people.)
Not murderers," I said, "serial killers."
"That's the difference between you and the rest of the world, John. We don't see a difference."
What the character says is, from a logical perspective, extremely stupid. There are different terms because people do see a difference. Everyone knows the difference. Serial killers kill multiple people at different times. Murders often were angry or drunk, or had something to gain, and kill one person.
You can go up to anyone and ask them how a serial killer is different from a murderer and they'll tell you. Everyone sees a difference.
The dialog makes sense from a social perspective. It's not a logical claim. It's a snappy retort. It doesn't actually work, but the intention is clear enough. And what does it matter if something works logically when it's adequate to communicate what a person cares about? The speaker doesn't care about logic, she cares about emotions and putting down John's deviance.
That's bad. It's irrational to focus on communicating emotions and allegiances, rather than ideas than make sense. It's not a truth-seeking way of life.
Going to the next step in the analysis, what does this passage reveal about the author? Well he could be stupid and not have noticed how illogical the claim is. Or he could be irrational and be the type of person to make statements like that for emotional and social reasons. Or he could think it's common and have written a flaw into the speaker on purpose.
The book is fiction. But the author either provides a real life example of an irrational character trait, or else he believes it's reasonably common (enough to make sense to readers) in real life.
That's exactly what we have to talk about," she said, watching me from the couch. "Your best friend's dad was murdered--seven people have been murdered in four months--and you're obviously not dealing with it very well. You've barely said a word to me since Christmas."This is the main character speaking with his mother. And it's another illogical statement.
"I've barely said a word to you since fourth grade."
"Then isn't it about time?" she asked, standing up.
Initially, her argument is that he's changed since Christmas. During the last four months of murders, he's been dealing with events poorly. Something changed, recent history is bad.
He makes a counter-argument that actually what she views as him coping badly is actually not a change. It's the status quo for years and years. It can't indicate some particular problem in the last four months. She's mistaken. Her argument is bad.
Then she drops the context. She ignores what she'd been saying and acts like his last statement was the beginning of the conversation. She takes it out of context and treats it as an isolated statement. Instead of treating it as a counter-argument, she fakes reality by pretending it was a general statement about his life. Instead of conceding the argument, she starts a new ad hoc argument. She has no respect for truth. She's just trying whatever angle she can think of to see if it works, without caring if she contradicts herself or changes her story midway.
She makes an abrupt topic change. First, he was supposed to talk to her more because of the last four months. Now, he's supposed to talk to her more because of the last five years. And her new claim doesn't make sense. During a time of turmoil, murders and temporary problems is not "about time" to change their status quo and break their routines. That's a terrible time to do it.
And, again, connecting this to real life: basically either the author is a bad person (the type to do this, or to think it's OK to do it), or he thinks tons of other people are bad people.