I think, on an etiquette board, the most important thing is what the person actually says or does. Because etiquette has nothing to do with thoughts or feelings, only with words and actions.
However. In the course of a thread, if someone tells me they think a person is a jerk or is rude or is stupid for doing something that is (to me) completely understandable, I'll explain why I think so. For instance, our upstairs neighbors are extremely stompy. I'm trying hard not to let it bother me, but the ill will is brewing. If I met someone else from our apartment complex and mentioned it, there would be absolutely nothing wrong with them saying, "Well, the floors are paper-thin. It's almost impossible not to sound like you're stomping in here, unless you tiptoe in bare feet."
If someone on here expresses a vehement disdain for another person, I think we ought to say something if we have another perspective. Going on the attack is out of line, but I do think it's perfectly fine to say, "I think you're being uncharitable, and here's why." This is an etiquette board, yes, but we often have threads that meander a little past the sharp, clear lines of politeness and rudeness.
ETA: It's also very hard to hide such strong emotions. Often, if someone is posting about how much trouble she's having with someone she thinks is stupid/low class/irritating/whatever, it occurs to me that her disdain is what's provoking some of the conflict, even if she thinks she's hiding it well.