Well, I was inspired. This is the first in a series of articles entitled "Bad Ideas," about, well, bad ideas, and explaining why these ideas are, in fact, bad ideas. I am well aware that the majority of the forum's users are enlightened as to the fact that these ideas are bad ideas, but they may not know exactly why, and these articles will also be for newbies who, being new to the game, simply are not aware that these ideas are unequivocally
bad ones.
So, without further ado, we move onto the idea of Chaos Tau.
What is the idea?Chaos Tau. This means, broadly, Tau who have been possessed by daemons or Tau that worship the Chaos Gods. It can
also mean Tau that have been press-ganged into service by other Chaos-serving factions.
Why is it a bad idea?We'll start with the "possessed by Daemons" angle. It is well established that the Tau have little-to-no presence in the warp (technically this means they have no soul, but that's for another day). As such it is
extremely difficult for a Daemon to possess a Tau, they must go through a much greater effort to get into a Tau's head than it takes to get into the mind of a human. This effort is, quite bluntly, not worth it. The Tau Empire is
tiny on a
galactic scale, and considering how hard it is to possess a Tau the influence gained compared to the effort expended it is not a risk any Daemon of any God is going to take. It is much easier to corrupt a human, and by and large the humans run the galaxy. No matter which Tau happens to be possessed, even Aun'va, it's going to be more worthwhile in the long run to take over a human in control of a significant Imperial planet. Remember also that Chaos kind of has a
vendetta against the Imperium and the Eldar, significantly less so with the Tau. Revenge is not really a motive.
The "possessed by Daemons" angle dovetails nicely into the "worshipping the Gods" angle, as the easiest way to get a
lot of people worshipping Chaos is to corrupt/possess an influential individual who then directs those under his influence toward worship. The Tau have a "shoot-on-sight" policy with all things connected to the warp, so if an 'O from any caste starts directing those under him toward worship he's going to be found out pretty quickly, and I'd imagine euthanised. Remember as well as lording it over the other castes, the Ethereal caste polices itself so if an Ethereal got mindraped by daemons they
too would be set upon.
What if a cadre was separated from the Empire, you ask? The Tau Empire is an atheistic society, so they are not going to be praying to any kind of God for their salvation when they are in danger. They do their duty. That's the long and short of it. Because of their utmost faith in the Greater Good, they aren't afraid of death in the same way that humans are. It's entirely possible (in fact, probably likely) that a Guard regiment in the grip of a debilitating fatal plague will all turn to Nurgle, but not the case for the Tau.
So we've established that the Tau don't get possessed (waste of effort on the part of the daemon) and don't worship of their own accord (shoot-on-sight warp tainted policy/atheistic society), and these are well known ideas. So now we're going to talk about Chaos Space Marines.
Another option to those that accept the reasons I've given so far is "They were captured by Chaos Space Marines and forced to work for them."
Why? Chaos Space Marines capture for two reasons: Forced Labour and Ritual Sacrifice. They don't get their underlings to fight with them except as meatshields, and Tau forces are no more likely to desert the Greater Good in the company of CSMs than they are anywhere else. It is most likely that the Tau, being weaker than humans and unlikely to do anything useful in the eyes of the prideful Chaos Space Marines, would simply be slaughtered as more bodies for the Dark Gods. It's not going to happen that way, boys and girls.
Those are the main ideas. If you have any
other ways in which Tau could fall to Chaos, please let me know and I'll debunk them too.
I really like the aesthetic, what do I do?Well, unlike most of these articles where the "I still like the idea, what do I do?" section is going to be "There's nothing you can do, suck it up," there
is something you can do here. The Tau Army, as a rule (and not a guideline) across the Empire, does not employ terror tactics. It doesn't see the point. They are practical. They do not display trophies of their foes; the closest to that they may get is I would not be surprised to learn of Tigershark pilots painting little Titans and tanks on their fuselage to represent kills. But the fact is breakaway factions such as the Farsight Enclaves exist, as do cadres embroiled in constant war.
Of course, this is constant war from a Tau point of view not from an Imperial one, so it's a fluid hit and run conflict rather than a war of attrition, but it still counts. In situations such as these it is plausible that the Tau involved might prey on the superstitions of their foes, say Imperial or Orks. This could result in battlesuits and Fire Warriors with armour modified to look gruesome, say with horns or bestial faces (giving the
aesthetic of Chaos Tau without any of the bad idea Kroot-
shtlk I've just gone through).
Just bear in mind that when they got back to the Empire (assuming that they're not a true breakaway faction), they'd probably be disciplined and issued with new, clean equipment

So that's the end of Bad Ideas Part I. I hope it cleared some things up for the newbies among you (that's not meant to be a derogative term, "noob" and "newb" mean different things), and maybe even for the vets among us who know that Chaos Tau are a no-no but don't know
why.If I've just gone on and on for ten paragraphs about something everyone already knows then... oh well

After a new post in the "Daemonic Tau" thread (which spurred this in the first place), I update this post with this:
ok guys we've established you think its a bad idea....
because i like new ideas, i'm gonna give you a new idea that is more plausible fluff wise 
Tau are a young a naiive race right?, and 'The Greater Good' is just a phrase, what if 'The Greater Good' meant serving a chaos god?, what if a cadre was decieved by a daemon?, Tzeentch is probably your best choice for decieivement here 
there you are, bingo chaos tau 
your army your models is the way i see it, you dont have to listen to these guys 
"Being deceived by a Daemon" does not mean Chaos Tau. Yes, a Lord of Change (or even a Keeper of Secrets) could appear as an Ethereal and convince a Tau Cadre to further its goals in the region, if it wasn't found out.
However:
1) If it was found out (and I BET Tau know if an Ethereal is an Ethereal or something else just pretending to be, they aren't just Tau with jewels in their heads) it would be destroyed. And the Tau are not naiive about the warp anymore, not since Medusa V.
2) If it does work (which, as I said above, is not something I believe likey) and Lord of Change-Ethereal says to Hunter Cadre 68B "Go and attack the Space Marines on this world", and they do it, that does not mean the Tau go all Chaos-y and mutate-y and start worshipping Tzeentch. If, when they attack said planet, a host of daemons appears you can bet that'll result in the Space Marines and the Tau actually joining forces temporarily to kill the daemons, because nothing unites warring factions like Daemons/Tyranids/Necrons.
Your suggestion, while slightly more plausible, does not actually give him (or anyone else) any license to produce Chaos Tau.
And I'm adding this response to my first "Bad Ideas" article.