The narrator begins by talking about Nietzsche's idea of "eternal return," the theory that everything we experience has happened an infinite number of times already and will continue to repeat itself infinitely.
From this perspective, a life that happens only once and then disappears is meaningless and without weight.
When something is transitory, we can't take it seriously, and so we can't judge it as though it matters.
This makes for a pretty cynical world, where everything is permitted because it happens only once and therefore cannot be judged. The narrator cites Hitler as an example of this sort of "moral perversity."