The Borderlanders offer to meet in Far Madding was insulting, yes. But since they have made no overt moves to attack him in the rear, why does he have to destroy them? And exactly what is the role of the common soldiers in the pissing contest between Rand and those rulers?

If Rand does suspect them of evil intent, he should kill them, and then allow their soldiers to go defend their homeland.

And why destroy them? Why not weave a massive burst of light, or a thunderstorm, or anything else that doesn't cause damage, but still sends a clear message that he can kill them if he wants? Then he can demand they come outside, and speak with him.

Or why not enter with Nynaeve? She has a well, so she will have the unique ability to channel inside. He could have easily used that to scare the rulers into following him, if he felt incapable of reasoning them into it.

Killing innocent soldiers who aren't even dreaming of threatening him because he feels pissed at their bosses for suggesting that they meet where he cannot force them into anything is nothing but an atrocity. Thank god Nynaeve stopped him.