Because despite his being (probably) the first Austro-Hungarian convert to Buddhism, Neumann remained culturally a Habsburg Catholic and this had much influence on the choice of diction in his translations.
In all Neumann has three ways of translating Māra according to what he supposes it to mean in each context. He uses “Māro der Böse” – when he thinks it’s the deva of this name (māra-devaputta); the Latin loanword “Mortem” when he thinks it’s being used as a personification of death (maccu-māra); and “die Natur” when he thinks it’s a personification of mental defilements (kilesa-māra)
Mostly his judgment seems fairly sound, but here and there we find him making some eccentric choices. For example, in the Bahudhātukasutta where the Buddha speaks of the impossibility of Māra being female, Neumann bizarrely decides to treat it as kilesa-māra rather than Māra-devaputta:
Er weiß: ›Unmöglich ist es und kann nicht sein, dass das Weib Herrschaft über den Himmel, Herrschaft über die Natur, Herrschaft über die Geister erlangen mag: ein solcher Fall kommt nicht vor.‹
But why refer to kilesas as “die Natur”? I suspect what he had in mind was an opposition that would have been very familiar to his contemporaries, but is much less heard of today, namely that between Heiligkeit and Natur. In this context the former refers to the prelapsarian state of holiness, innocence and harmony, and the latter to the corrupted and fallen nature that obtained after the talking snake incident in Genesis 3.
“Indem Adam und Eva dem Versucher nachgeben, begehen sie eine persönliche Sünde, aber diese Sünde trifft die Menschennatur, die sie in der Folge im gefallenen Zustand weitergeben. Sie ist eine Sünde, die durch Fortpflanzung an die ganze Menschheit weitergegeben wird, nämlich durch die Weitergabe einer menschlichen Natur, die der ursprünglichen Heiligkeit und Gerechtigkeit ermangelt.”
Katechismus der Katholischen Kirche, (1997) Nr. 404.
Nowadays the translation doesn’t really work, for even though plenty of Christians still believe in original sin, the concept no longer has anything close to the extensive cultural influence that it did in Neumann’s day.