kaiba spoilers, armchair philosophy
kaiba's ending is an assertion that gnostic, hegelian idealism fails to account for the material conditions we live in, that it does not "respect" the world for what it is and how it shapes us, and that following universal spirituality without acknowledging the material world may very well lead us to unreasonably justified mutual destruction.
the early episodes show us ways our material conditions impact our identities and individuality to underpin this.