I think it's equal tbh. Moscow helped Germany skirt restrictions on mars manufacturing in the build up to the war as well. I personally hold them jointly responsible as they both triggered the defence pact for Poland and the USSR invaded and occupied European territory at a similar rate.
It didn't happen in a vacuum, this was at a time when Hitler also annexed Austria and the Sudetenland. Of the two, Hitler was more rattling the cage and setting the stage for war.
Of course. I didn't say he was solely responsible but Stalin was absolutely culpable for starting WWII