It's pretty clear that, at the start of the series, Uncle Six is pretty evil. Besides the minor fact of being a crime lord, he also has a habit of incinerating his own lackeys, rounding up random chefs to be interrogated or killed, and so forth. However, was he always this way?
The first Wu assassin certainly seems to think that it's intrinsically bad to have a Wu piece; similarly, Uncle Six refers to seeing the fire behind his eyes in a fashion that implies that it exerts a negative influence on him. Not to mention that Uncle Six himself wants it out of him, and that they perform an exorcism of sorts to remove it, during which he says things in a demonic voice. On the other hand, Kai says that it is what people do with the Wu pieces that is evil, not the pieces themselves, perhaps suggesting that it is people's own flaws that tempt them to evil.
So, what made Uncle Six so evil? The Wu piece, or his own demons?