This is too complex to answer with a few sentences but I can make a feeble attempt ...
1.WW2 was in many ways a result of the way WW1 ended - Partially true but not quite, when Germany capitulated it came as a big surprise to both it's citizens and it's soldiers and they never came to term with "loosing" the conflict, the German populace suffered greatly both from the declared loss and the aftermath of a ruined conflict.
2.One popular statement is: Why wasn't Hitler stopped before the war? - Actually most people both inside and outside of Germany thought at the time that most of claims were valid claims, the merger of Austria and Germany was welcomed by the Austrian people that really struggled finding their own identity after Austria-Hungary was divided after WW1. Germany lost a huge chunk of land both to Poland, France and the former republic of Tjekkoslovakia that many French and British politicians thought was rightful to reclaim at the time.
3.Could it have been avoided? and could it have been a shorter war? - I believe it couldn't, it came to be under very special circumstances and if it wasn't Hitler it would prolly have been another ... Stalin, Roosevelt and Churchill was very specific that the war had to end the way it did, the surprise of the ending of WW1 was not to be repeated, Germany had to loose and by a clear margin to prevent WW3 to spark off some odd years later.
I recommend this book for in-dept knowledge of the Nazis, how they came to be and how they steered Germany into the abyss.
Wait behind the line ..............................................................