Power reveals what politeness sometimes hides.

Manners work well while incentives are aligned. Give someone the ability to act without cost, and what emerges was usually there all along, held in place by the need to be agreeable rather than by anything internal. Power does not corrupt so much as it removes the reason to conceal.

Watch how people behave when they have nothing to lose from behaving badly, including in small things. That is better evidence than any amount of pleasantness under pressure.