D had always been pondering how to destroy this world.
Of course, he wasn't talking about destruction in a physical sense; in fact, even if D detonated nuclear bombs turning the planet into wasteland or obliterated it into pieces, he wouldn't destroy this world. On the contrary, such actions would merely "solidify" the reality of "human destruction" in this world, manifesting it visibly.
Thus, destroying the world wasn't about physical destruction; it necessitated considering the issue from a different dimension.
