Born in Steyr, Austria, after finishing middle school, Eigruber underwent training in geodesy and fine mechanics at the Austrian Federal Teaching Institution for Iron and Steelworking.

When Passau Nazis met at the Deutschmeister Inn, Eigruber joined his allies and reminisced about their venture during the Kampfjahre.

There, he would be ceremonially welcomed at City Hall, and attend the latest play by Hans Baumann.

In response, SS soldiers, Sturmabteilung detachments, and Volkssturm men, who were acting on Eigruber's orders, hunted the POWs down in the Mühlviertler Hasenjagd.

As for those who were captured, a witness said they heard Eigruber telling commandant Franz Ziereis that "All these pigs will have to be finished.

In April 1945, shortly before the arrival of U.S. forces, he ordered the mass executions of 42 resistance fighters.

He managed to hide out using forged identification papers until 11 August 1945, but was arrested by US Army Counterintelligence, in Sankt Pankraz (a few miles south of Kirchdorf an der Krems).