As there are great answers regarding the dim light, this is only about people are meaner.
tl;dr: Cultures are self-enforcing and self-sustaining: Find an initial trigger that caused mistrust in society and you'll generate potentially very mean and egoistic people.
Behavior of individuals in society is a function of the culture of that society, and culture is the summary of all behaviors. Sociological and economic research has shown (in actual lab experiments, if I find them) that in standard trading games people behave nicely if everyone else does, too.
The summary of the experiment is roughly this: 10 (or so) players get randomly coupled up and will play a Game where you can choose either a "coop" or "egoistic" action. If both play coop, both get high outcomes. If one plays coop and one egoistic, the coop player gets nothing and the egoistic player gets more than in the previous case. If both play egoistic, both get very very little.
Notice that this game represents some important feature of culture: If everyone plays nice, the total outcome will be the highest. Mean people/free riders will benefit (earn more from free riding), but only if the remainder of society is nice. If everyone is mean, nobody profits. But also: If everyone is mean, it is the optimal strategy to be mean.
In the lab experiments, you get repeatedly matched with new people. You don't know how they played individually in the last rounds, but you know whether the total group of people was doing well or not. What happened there was that some groups by change ("inexplicably") turned into coop groups where everyone played nice, and some deteriorated. One of the determinants was group size: The larger the group, the more likely it deteriorates (similar as how people in smaller communities are more trusting than in large cities). Moreover: Move some from a "coop group" to a "egoistic group", and they will slowly learn and adjust.
So cultures are self-sustaining: Find an initial cause that made the culture to be bad and egoistic, and you'll find a self-enforcing mechanism. An initial group of mean people, perhaps followers of Evil Guy? At his arrival, Evil Guy Ponzi schemed the heck out of everyone and they are all now sad and lost and trust no-one? Perhaps in your world people look all very similar (dim light), so it's difficult to build up trust?
All of these mechanisms would be removed with the dimming: If Evil Guy is defeated, public information that his followers have left could turn society, if the cause was dim light so could the change of atmosphere.