Hitler was an Austrian World War I veteran who became chancellor of Germany and leader of the Nazi Party. His racist beliefs and boundless desire for militant expansion led directly to World War II and the Holocaust. Although his official title was Chancellor, he was referred to as "Führer" (literally "Leader") by the German people, and he was one of history's most infamous dictators. He committed suicide in 1945, but the true story of how and why was not known until many years after the war.
Sometime prior to 1945, in a bid to win the war after the Wehrmacht was being trounced by the Soviets in Russia, Hitler turned to the Vrilerinnen, a group of human-alien hybrids who were extremely ancient and shared his vision of a master race. They offered to use their advanced technology to assist the Nazis in winning the war. Hitler gave them a high-tech research facility on Rügen Island, staffed by German scientists and Waffen-SS troops commanded by Hitler's loyal subordinate General Jürgen Hoff.
When the tide of war turned against Germany despite the Vrilerinnen's involvement, their leader Maria turned against Hitler and took over Rügen Island. She decided the Nazis weren't the allies her people needed, and instead turned to Duke Nukem's old foe, the Cycloid Emperor. His alien army occupied Rügen Island. Except for a few SS troops and scientists, the aliens killed all the humans present. Maria herself shot General Hoff. Hitler remained unaware of this betrayal, however.
In the future, an elderly former French Resistance fighter named Elise Plewmann contacted Duke Nukem, fresh from saving the world from the Cycloid Emperor and his army again, and explained he needed to go back in time to help stop the Vrilerinnen. After some convincing, he did travel back where he joined up with the young Elise and her companions, Englishman Major-General Conner Sean and American Corporal Flip Henry. Although both Sean and Henry died in the process, the group was able to successfully attack Rügen Island, killing all of the Vrilerinnen, Nazis and aliens.
Before returning to his own time, however, Duke, who had once visited modern Germany and considered it a nice country, decided that he thought the Nazis "stank worse than the aliens" and went to Berlin to personally kill Hitler. Entering Hitler's office, he confronted the dictator who had brought Germany and most of Europe to ruin and ordered the deaths of millions, and betrayed the human race by allying himself with the Vrilerinnen.
"Yo, Shitler!" Duke challenged.
"Ach, scheisse!" Hitler cried upon seeing Duke. He then shot himself in the head.
"Pussy," a disappointed Duke commented before travelling back to his own time.