Depends on your viewpoint. According to the Aztecs, he was the god of discord and deceit as well as the god of robbers, but he was also the god of rulers, warriors and sorcery. He was associated with the notion of destiny or fate and was known for inciting wars between peoples.
Him and his twin brother Quetzalcoatl were considered enemies, with Tez being the more sinister. Thus in one story when Quetz tried to knock Tez out of the sky, Tez turned into a jaguar and destroyed the world. Following another effort by Quetz to rule the world and repopulate it, Tez devestated it again and turned all the surviving men into monkeys. But Quetz had his own revenge when he destroyed the world with fire and turned all the survivors into birds.
So it's difficult to say either way really as his brother was just as bad, and other gods subsequently either flooded the world or turned the survivors into other things such as fish etc.
Overall, being a god who is also known as the "Sower of Discord on Both Sides" (Necocyaotl) one would consider that Tez was a bit of a bad god unless you were an ambitious ruler.