If Hitler had not existed, Germany would not have recovered from the depression when it did. And likely the USSR would have invaded Eastern Europe, triggering WW2 with themselves as the enemy instead of an Ally. With Germany unprepared for war it would have been a fairly easy conquest, and France and England would have been even less ready they were historically without the growing threat from Germany.
So, likely Russia would have conquered most of Europe, with the possible exception of England. Russia would not have declared war on the USA after Pearl Harbour, and the USA might have stayed out of the war in Europe to deal exclusively with Japan.
So, in the end, the war would still have happened, just not the same way.