Why did Ralf (Bruno's dad) join the soldiers and become a commandant?

Bruno's father Ralf is a career soldier. He fought in World War One, and like many German veterans, was drawn towards the Nazis, as he wanted to rediscover the camaraderie and sense of purpose he left behind in the trenches. He's a very ambitious man, and has risen swiftly through the ranks. The position of Commandant at Auschwitz is simply the latest in a long list of increasingly important posts to which Ralf has been appointed.
As well as being fiercely ambitious and career-minded, Ralf's also a fanatical Nazi. He knows full well for what heinous purpose Auschwitz has been established; he knows that it's a factory of death, in which countless men, women, and children will be murdered simply for being Jewish. Yet he has no hesitation in accepting his commission. As far as he's concerned, he's doing the right thing by his Führer and his country. He sees taking up the role as Commandant at Auschwitz as no different from serving in the trenches during World War One. But Ralf's mother couldn't disagree more; she tells her son to his face that she's ashamed of him for agreeing to take on such a terrible role in the Nazi system.