You can review this scene if you open the book to about 1/6 of the way in, then look for the chapter titled "Tuesday, September 5." You can see there how Paul's reaction to Mike's sudden death is very normal, involving shock, disbelief, and caring concern for Mike's family. Understanding Paul's reaction, and contrasting it with Erik's reaction, is a good way to understand how Erik's is vicious and inappropriate. Let's consider the details:
Paul's very first reaction to Mike's death, when he hears about it, is a motionless state of shock. Paul stands perfectly still, staring at his brother Erik who'd just revealed the news; even though Paul is holding heavy grocery bags, he doesn't set them down or move to carry them into the house. Instead, he remains speechless and motionless while his mom struggles to speak to Erik; she's trying to wrap her mind around the concept that a boy who had just visited the Fisher home had been suddenly killed by a lightning strike on the football field.
When Paul gets over his initial shock, his next reaction to Mike's death is to ask his mother if she thinks it would be a good idea to call Joey. Joey is Mike's brother, and a close friend of Paul's. But at his mother's suggestion, Paul waits. It's better to not disturb that family while they're probably in a state of severe shock themselves, she implies, and Paul silently agrees.
After a few more minutes pass, Paul's reaction to Mike's death starts to focus on Erik and Arthur. How could they possibly be laughing about how Mike died, and about how Mike's little brother lost control of himself as a result? Paul's emotional state now is disgust and anger.