Why does one generation have to undergo such terrible cataclysmic bowl judgments?
That generation undergoes the cataclysmic bowl judgments because it brings to a head the cumulative sin that previous generations sowed. Having shed the blood of saints and prophets, it receives what is deserved, and God's judgments are true and righteous. Scripture shows God visiting iniquity to the third and fourth generation.
Les explains that people naturally think God is not being fair when one generation faces such terrible cataclysmic events. But that generation is simply bringing to a head everything previous generations have been sowing the seeds of. God has always done that, even if we cannot always reconcile it; He brings judgment and chastisement on this generation for what three or four generations back were guilty of. It is just the way God does it, as in the Ten Commandments where He says He will visit down to the third and fourth generation. The context also notes they shed the blood of saints and prophets, so they were given blood to drink and were worthy; the altar declares God's judgments true and righteous. The fourth angel then pours his bowl on the sun to scorch men with fire.