If the people suffering don't die their suffering is prolonged. The body breaks down. It is painful, death is mercy.
And that is the point he is actually trying to make. There have been documented cases where someone has been fatally injured in such a way that they were looking at a lingering (as in, weeks-long), excruciatingly painful death (for example, having their abdomen punctured, especially if it cut or severed the intestines, before the advent of antibiotics). In those cases, other people would very often give them a coup de grace so that they would not suffer needlessly. But it never happens as an act of divine grace. They never simply die painlessly a few seconds or minutes or hours after suffering that injury. No, even if they're devoutly praying for death, it doesn't come until they've weakened to the point where their body's systems simply can't keep them alive any longer - unless a person gives them a mercy killing.
Answer me a question please. If there is an afterlife of the spectacular kind, is dying such a terrible thing? Yes or no please.
Yes. Because it is that belief that there might be a "spectacular afterlife" that fuels the fear of death, and which makes it so terrible. In actual fact, death is nothing special - simply the cessation of life functions. Every atom is still there, and it will continue to exist as part of something else. That's not really all that bad, especially if the things you did are remembered by other people.
No, the idea of death being terrible came directly from the belief that there might be life after death - but only for the ones that God thinks is worthy of it. How many people do you think worry about whether they're one of God's chosen who will live on after death? How many do you think wonder if they aren't destined for some other fate? How much suffering do you think they put themselves through during their lives because they aren't sure if they'll live on?