[@Kingfisher]

Oh, he would know full well that the cultists aren't the good guys by any stretch of the imagination. But the Tau Empire needs to get a first hand account of what exactly this insanity is so that they aren't taken by surprise by it when they encounter it themselves... and if the price for this knowledge is paid by the Imperium of Man rather then the Tau themselves, all the better. If the Imperium is weakened by this 'Black Crusade', then the Tau Empire will have an easier time expanding and defending its boarders while spreading the light of the greater good. The needs of the Tau'va and the Tau Empire come before the personal ambitions, desires, lives and morality of the individual after all...

And since Tzeentch is the Chaos God most likely to give Fio'Ui the time of day let alone his blessings, is it really that strange that Tzeentch would be subtle about guiding [i]anyone[/i] down the path he wanted them to go? Worst case scenario, Fio'Ui dies early on and Tzeentch doesn't really gain anything from this dead end of a plot. He doesn't really lose anything either though... 

It'll be interesting to see what kind of possible plans that Tzeentch would want to set in motion by using a tau... I could even share a couple I can think of via PM if you want.