User blog comment:B214/Question and Answers 6/@comment-106.68.232.55-20161011071720/@comment-9595671-20161021230303

He actually does cause the end of the world, because the moment he lets go of his tail is the moment Ragnarok officially begins, his page on the wiki even mentions this. The DxD world just replaces releasing his tail with waking from his sleep as the catalyst to Ragnarok's start.

So he still causes the ends the world, he's just the catalyst to making it start in the first place, not the one that destroys it. It's no different to how the meteor that struck Earth during the Dinosaurs' era didn't wipe them all out upon impact, but the effects caused by it still made it responsible for killing them.