The Russians hated the Germans on account of the atrocities commited against them by the Germans.
When at first the Germans invaded Russia, many Russians greeted them as saviours. They didn't think for a moment that the Germans would mistreat them, as they saw the the Germans as being an old cultivated and civilised people. As a result of this many of the Russians, having survived being taken prisoner by the Germans, were treated badly by their Russian liberators as Russia recovered occupied territories and, indeed, territories within Germany itself.
"Although God cannot alter the past, Historians can"