I thought the whole idea of Warhammer 40,000 is that there is no good guy, just different shades of misery and death.
That's true. Just different levels of evil. Then again, the Tau don't seem so evil.
If you ever have thought about Tau's one-ness, you'd find that it is kind of at odds with their militant nature. They're like Ghandi in really cheap Gundams and altruism isn't found at the end of a gun. They are the embodiment of that tyranny of the majority that start with everything good for you, until they take away all of your rights. They're kind of like 'progressives' in power armor. Tau may not be Chaos, but they are certainly more evil than Eldar or the Imperium as a whole.
Hmmm... You make an interesting point. My friends and I often muse at the principle of Tau communism, yet it really isn't far from the reality of the Tau empire. How long until their society becomes tyrannical and fascist?