A good rule of thumb for moderation is: if it's allowed, expect it to happen eventually.

**Why does moderation exist?**

This is the philosophical question which drives the answer to your question. The answer, in short, and however each community defines it, is: **to prevent things from getting out of hand.**

Of course, each community defines this and sees this differently. Forum culture, for instance, might not consider noise replies ("Thanks!") 'getting out of hand,' whereas Stack Exchange does. Each subreddit defines their own rules of moderation. 

**When you introduce an unmoderated section, you are telling people that everything goes.** In short: yes, you have to expect things like this to happen. The fact that the 'authority' has created a moderation-free space implies that it's totally okay with administrators for users to go and do/say whatever they want, when in reality, it _very much isn't_. 

In other words, if you have an unmoderated section, you are allowing everything. Death threats, harassment, violence... all of it becomes acceptable in this one little space. So, when do moderators step in? 

Draw the line somewhere. It's okay to have a section which is less stringent about content and topic control, since this allows people to vent some frustration and/or post off-topic content, but make sure you know where your boundaries are and strictly adhere to them.