Discourse Norms: Moderators Must Not Bully
One of the absolute worst things that can happen to a civic/public community online is for moderators to be bullies or for moderators to take the side of the bullies. Once that happens, the community is at grave risk of ceasing to be a public community and instead embracing cliquism. If the moderators enforce the will of their friends rather than good discussion norms, the space is no longer going to be a space for good discussion but rather one for a certain friend group.
The most common way I’ve seen this happen goes something like this. A newcomer with locally unusual ideas joins the community. Conflict between their ideas and the more established norms arises. Because these ideas are unpopular, people push back against them, often in mean or uncharitable ways. If left unchecked, the newcomer may soon become a target of bullying and sniping. 
At this point, moderators need to intervene in favor of the newcomer, because mean and uncharitable behavior shouldn’t be allowed to stand in a civic/public space, even if it’s towards ideas that are locally unpopular. Moderation is needed to rein in the attacks and keep things civil and productive. However, in practice what often ends up happening is that the moderators intervene against the newcomer, enforcing the local social hierarchy rather than good discussion norms.
This is toxic to a civic/public space and, if left unchecked, drives out views or discussion styles other than those that are locally popular.
One potential antidote to this sort of behavior is holding moderators to significantly higher standards than users. If a moderator and a user are in an angry, insulting argument with one another, the moderator should be removed from moderation or at minimum recuse themselves. If a moderator posts insults against another user—especially someone who isn’t popular—they are at fault and should apologize or be removed from moderation.
Yes, this is a harsh standard. Yes, this means that being a moderator limits what you can say in some circumstances. But that’s what you need to do to keep the bullies at bay, and ultimately, being a moderator shouldn’t be a position of power but rather a position of responsibility.
Lastly, I want to point out that it’s totally fine for a space to exist for a friend group or for those who agree with certain perspectives—and for those sorts of spaces, it’s entirely fine for moderators to enforce local social norms or locally popular opinions! However, there’s a big difference between that and a civic/public space, and if you’re going for civic/public norms a higher standard is needed of moderators.
 This obviously doesn’t apply to Nazis and the like, which should IMO be banned outright.
 Note that footnote  should not be construed as an excuse to go around calling everyone you don’t like a Nazi in hopes of getting them banned, and such rules should be clearly articulated beforehand—the intent is merely to point out that you can have a civic/public space that still prevents certain objectionable content.