I think one of the most disgusting moments was the opium trade and wars in China.
I could add in Poland there is a concept of so called 'Western Betrayal' about WWII.
Several events account for the concept of the Western Betrayal:
1. The Munich Pact. The destruction of Chechoslovakia forced the Polish command to adopt a defense plan that assumed the long borders must be protected, instead of forming a central defense ring. The fear was that if Poland gave up the western lands without a fight, the Reich would then stop military action and the Allies would agree for another pact like the Munich Pact. However Polish authorities were partially guilty to this, as when Czechoslovakia was annexed, they also send an army to annex some disputed territory instead of backing up Czechoslovakia and opposing the pact.
2. Mobilization was announced on 30 August and called off under the pressure of the Allies. Together with generally poor mobilisation plan, this made the Polish Army to stand up against the invasion having 30% fewer troops that it could have had, and with many units concentrated at random places.
3. The agreements made at Yalta conference were openly contrary to the insterests of Poland and to the safety of its citizens. For these reasons it's also called 'Yalta Betrayal'
4. Only a cherry on top after selling Eastern Europe to the USSR, was that the Polish soldiers were not allowed to take part in the victory parade in the UK. Those who came back to Poland were killed, and those who stayed in the West did not receive any help from the British government. Even generals and heroes like general Sosabowski and Maczek had to earn their living working as low rank blue collar workers in factories and warehouses, or as bar tenders and so on.
The memory of this is vivid to this day, and among other historical events helps feeding Siege mentality that is cynically used by local politicians to do whatever they want and exploit the nation.