If Hitler had been killed before WW2 started, could the war have been avoided?
Hitler's death would probably have led to Goering's accession. Goering would have done much to avoid a war with Britain and France. He would probably not have torn up the Munich Agreement. Rather, he would have pressured the remnant of Czechoslovakia into agreeing to so many German demands that it would become a puppet state. Goering could live with a Czech puppet state.
At some point, Goering would probably have demanded that Poland turn over Danzig and part of the Polish Corridor to Germany. But if Poland refused his demands, he would probably have arranged matters so that Germany would not appear threatening to Britain and France, and that they would not ally themselves with Poland. So, any war would be an isolated German-Polish war.
Goering was most interested in making money and living a luxurious lifestyle. So, he was far more interested in growing but peaceful German domination of Europe, as opposed to war with other great powers and its risks.
Not entirely. The European Theater could have been a much easier fight because it would only be Mussolini and perhaps Franco, although the Soviet Union wouldn't get involved because they wouldn't be threatened by a rapidly expanding fascist regime on their border. The Japanese still would have expanded very aggressively in the Pacific, but they would likely become the main focus of the war instead of Germany. In fact, Germany may have even fought with the Allies, although that would be unlikely due to restrictions. Looking at US involvement, it depends on when the Japanese attack a US territory, whether or not it's like the massive surprise offensive seen in our timeline.