I've never met Zed. However, from the rant and from reading the interview with Zed about Mongrel a couple years back, I think Zed is being true to himself. To me, that's more important than crossing socially sanctioned lines ... even if it is name-calling.
I also know that I'm in the minority here. I value self-honesty more than appearances of professionalism or maturity. Self-honesty is extremely difficult to accomplish, if you can accomplish it at all. I get the sense that Zed prizes this as well. Won't really know unless I've met him.
Sure, but aren't these things orthogonal? Basic professionalism and maturity need not be excluded because of self-honesty.
He threw a tantrum in this rant, no bones about it. But I'm not going to make a judgment on whether he was justified or not. Zed just is, this is the choice he made, these are his actions, and there will be ripples from his actions. So be it.
But the point is that just because it was a bad idea, doesn't mean Koz is categorically a bad developer, or that he's any of the things that Zed called him. I heard him talk at RailsConf, and I have to admit that he sounded kind of like an egotist, but didn't actually know how to code all that well. But do I call him a fat fucker? No. That's taking it a little too far.