According to the British Library - Atrocity Propaganda in WW1 focused on the most violent acts committed by the German and Austro-Hungarian armies, emphasising their barbarity and providing justification for the conflict.
Investigations into the nature of war propaganda suggested that atrocity stories had been fabricated by the Allies in order to justify the war and to encourage enlistment.
https://www.bl.uk/world-war-one/articles/atrocity-propaganda#
They LIED about the GERMANS in WW1.
They LIED about the VIETNAMESE
They LIED about IRAQ
They are LYING about TRUMP + Supporters.
What are the odds that they lied about the NAZIs in WW2?
This is a little half and half. Throughout history terror could intimidate. If your city resisted it'd be destroyed to show the other cities not to resist. If your city surrendered you could be merciful and treat them well, convincing other cities to also surrender in advance.
As for WW1 the initial waves of troops going through Belgium did not have time to be gentle, and believed a few harsh reprisals now would prevent further resistance. In WW2 the Germans outright had atrocity as a "logical" policy. If they are going to blockade us, and we have millions of extra mouths to feed, they took that option. Tokyo had trouble obtaining food before their war started.
There is often a reason someone would lie or a cause to make them exaggerate.
Short end of the story, people Lie about their enemies.