Indeed, and the work of several hours is appreciated. 
"Beevor argues that these acts of violence were initially motivated by a desire for revenge and retribution for crimes committed by the Nazis during their invasion of the Soviet Union" 
I would suggest this small clause would cover the subsequent changes to the patterns of violence, sublimating any ideological, propagandistic element which was to a considerable extent evident in the initial phases. A reversion to the age-old behaviours of fighting men in the susequent phases could be included.