It appears to be part of the German national character.
Barbara Tuchman, in The Guns of August, notes that Germans were filled with genuine moral indignation when Belgium resisted invasion.
I suspect that this arises from the manner in which Germany became unified and its lightning transformation into a "Great Power."
Prior to German unification, the collection of some 300 petty states which which were to become Germany were regarded as something of a joke. They were seen as a gaggle of petty princlings building copies of Versailles, strutting around in fancy uniforms, heaping honors on each other and generally making fools of themselves. Prussia had some influence and power but it was at the price of becoming, as was said at the time, not a State with an Army but an Army with a State.
The brilliant maneuvering of Bismarck which created the German Reich left the resulting state with a citizenry which felt the need to compensate for centuries of being "dissed."
The result was a nation and people who were pugnacious yet unsure of themselves, proud of some very real accomplishments and yet also filled with a sense unworthiness.
The result is the universally recognized German rudeness, sense of entitlement, and historic propensity for barbaric behavior.