Klarion's decision to make all the adults and children separate led to some unexpected fatalities which he had not notified a certain Master of Death ahead of time. This makes them a little more than angry. AKA Harry and Klarion have a deal that whenever Klarion does his schemes he would let Harry know ahead of time so he can prepare for the deaths since Chaos Lords can interrupt the flow of a person's life. Meaning someone could die later or earlier due to this and Death doesn't like this, so he doesn't like Klarion who causes this.