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.

Himmler killed himself right after being captured, before being investigated right after WWII. So, safe guess is that it is hypothesis. It can not possibly be after war reflection.

Himmler was horrified and mentally affected after mass shooting he seen. So, he started to figure out how to kill Jews differently, out of worry for mental health of German soldiers.

So, the above is at best him figuring out how to kill more effectively. Definitely not some kind of guilt or regret.