Russia showed willingness to go to war with Germany only after Hitler decided to not respect the deal he had made with Stalin for non-aggressiveness between the two countries.
Hitler decided that he will attack Russia, and so he did. Stalin was shocked because Hitler turned his back on him and found himself in a situation where he is under attack. All of the Russian army as well as millions of ordinary people were mobilized in order to stand up and defeat Germany. Russia managed to win eventually but it didn't stopped there, it went after the Germans to their territory and occupying part of it, killing lot of their soldiers and civilians in the process.
