Had World War II not happened, how might history view Hitler and his achievements in 1930s Germany?
I will assume that you also are supposing that the Holocaust would not have happened and that the persecution of Jews would have been kept to the relatively low levels seen in the 1930s. If so, I think that Hitler would have been seen as more of a success. This would depend, of course, on how his time in power ended.
Before the war started, Hitler had managed to help turn Germany around economically and in terms of national pride. If he had left office in 1938, for example, he would probably be remembered quite well. He would be seen as a man who had brought hope to a downtrodden country. He would have persecuted Jews, of course, but very few people whould have held that against him given how pervasive anti-Semitism was in the Western world up through WWII.
If Hitler had stepped down with German pride restored and without having gone to war or killed millions of Jews, he would probably be remembered fairly positively.