There are various reasons depending on the individual and the war:
-in WW1 many Germans thought the assassination by Archduke Franz Ferdinand was a conspiracy between the Serbian terrorist organizatiuon, Serbia, Russia and France to encirlce and destroy Germany and Austria-Hungary (later it turned out it was "only" a conspiracy between the Serbian government and its terrorist organization)
-then it was Britain that declared war on Germany, because the Brits were afraid Germany would stay on the Channel in Belgium
-also many Germans believe their propaganda (as did all nations)
-Germans were usually not in favor of the war and according to Ian Kershaw, they were only in favour of the war after France and the British Forces were defeated after the Battle of France.
-also there was no way that somebody could openly criticize Hitler. Those people who did were sent to Concentration camps (not only Jews) and even their family members and frieds felt repercussions. For instance, children of opponents of Nazis were sent to Nazi supporters who raised them. Hence, if you are a parent you think twoce before criticizing. And people who did not follow while in the military were either sent to the Eastern Front (where fighting was much more brutal) or killed.
Also: would you switch to another country, which wants to destroy your country in turn or turn guns against those people who could kill you as well? And killing Nazis also could have let your family members and friends being killed.
And there were many assassination attempts in Germany to kill Hitler.
Have you ever seen an American kill his/her president when the US went to war or a British person killing the PM to prevent / stop a war or from other nations?
And of course not all were good ro bad. And wants to judge anyway? Who knows all people and all the deeds they have done? And some people have even done bad and good deeds during the war.
-and agains as in WW2, many Germans believed propaganda as much as people from other nations