While it's still as true as ever that flags and close votes are not super downvotes (or even super delete votes; standard policy for moderators in response to "please delete this" flags is to decline), everyone, please do use flags when you come across things that shouldn't be on the site.
Particularly, if you come across content that you feel violates the be nice policy, flag it!
It's rare to come across rudeness on Worldbuilding, and honestly on Stack Exchange in general (being brusque to newcomers, while a problem, is a different problem), but especially on a reasonably high-traffic site such as Worldbuilding, it's pretty easy for stuff that really doesn't belong at all to fly under the radar. Once such content is brought to our attention somehow, we might then be looking at it in isolation; if people flag content that they feel is not-nice, then even if not every such flag ends up being validated as helpful, any pattern will be visible to moderators via the content owner's profile. This makes it far easier to accurately judge if it's a one-off thing, or if it's actually part of a pattern, which helps us to correctly handle the situation.
Moderators can't read every post and every comment, so we rely on the community to flag things that don't belong. Flagging is the mechanism of the Stack Exchange software to bring content to the attention of moderators, or to the attention of the community via the review queues. It's a feature that's available to just about everyone with at least a little bit of participation on the site; only 15 rep is required.
On the flip side, please be considerate with custom flags. Each custom flag (the one where you write something into the "in need of moderator attention" text box) goes directly to the moderator inbox and can only be handled by a diamond moderator. There absolutely are times when this is called for, but before you do that, please do consider if some other type of flag (possibly in combination with a clarifying comment) will fit the problem you're seeing with the content. Before raising a custom flag, ask yourself what, exactly, you want a moderator to do about the post, and whether that really requires a diamond moderator. (It's quite okay to flag posts to point out issues that have been fixed but are likely to be of legitimate interest to the moderator team, but if you do, please make it clear in the text of the flag that the issue has been resolved and you're flagging only for posterity.)
On the third side, when flagging comments specifically, please do consider providing more details in a custom flag, because comment flags go directly to the moderators anyway (so there's little difference in practice between a standard comment flag and a custom comment flag). It's difficult to handle the flag correctly when all we see is a comment in isolation and a flag that says "no longer needed". Instead, take a few seconds to type up a more descriptive reason for why you feel the comment should be removed. For example, we might see "has been incorporated into question" or "seems to reply to deleted comment" or even "answer in comment". (If your first language is something other than English, then don't worry about the finer points of linguistics. As long as it's clearly understandable, that's good enough.) Seeing such flags immediately tells the moderator handling them what the user flagging the comment felt was the reason why the comment should be deleted. The standard comment flags used to be more specific, clearly separating "not constructive" from "obsolete" from "too chatty", all of which are now bundled together as "no longer needed". Unfortunately, this makes the job of the moderator trying to handle that comment flag more difficult, because we might not see why the specific comment is "no longer needed".