

Simple material effect.
People are shaped by the interrest of the social and material class they are part of. And the evil people are always the one in power and have the money, so they act in the interest of their class and do things that help themselves.
Fucking Marx wrote about this quite a while ago, now his predictions just manifest more visibly, but the cultural and material forces have always been at play. Now we just reached a breaking point as a planet and population, so the underlying systems start to break too and the people on top do everything to stay in power, including ditching reality altogether.
If you want to judge the character of a person:
Observe how they treat those they have power over, usually in terms of social hirarchy or economic position.
Edit: Read the question wrong. Be nice to the service workers you rely on to get things done, like janitors, servers, cleaners, basically the bottom ranks of the totem pole.
If you have to ask why and need a selfish reason:
These people often are in positions where they can sabotage you, make your life difficult or slow you down. You really dont want to scream at the people who handle your importent paperwork, which can easily “get lost in the administration”…