Depends what kind of society you want to have. People can police each other. You’ll always have bad actors in society, but you may not want to be on the frontlines of policing them. Especially when you’re talking about groups of bad actors vs. an individual. Ideally, you would live in a society where there is economic and social equality and people don’t have a need to commit crimes upon each other, but that’s a fantasy world. You could have a society brainwashed by religion but that’s not one of equality depending on who decides the rules.
Small community, probably less need for police. The larger the population, the more difficult it becomes to manage crime.
You might be asking the “are people inherently good or bad” question and unfortunately the answer is people are human, and humans are animals.
Lemmy mods, at least in Lemmy World, often police the will of the community. If there is a user who much of the community disagrees with, whether they are being abusive or not, they will block them just to maintain the echo chamber. If that’s the kind of society you want, then you definitely need mods.