I think it's a reasonable inference from a speech Himmler gave at Posen in October 1943. That speech was both frank and well-documented, and it was used as evidence in the Nuremberg trials, so Arendt must have been familiar with it.

Himmler speaks of the challenge of remaining a decent person despite witnessing and committing atrocities (the section around “Dies durchgehalten zu haben, und dabei – abgesehen von menschlichen Ausnahmeschwächen – anständig geblieben zu sein”), showing the kind of reversal at work that Arendt describes.