The Innumerable Northern Serpents

Question: So what do the Centzonmimixcoa actually do in Aztec myth? Are they as antagonistic as the Huitznahua?

The Centzonmimixcoa, despite their more dragony and impressive name (literally "The Four Hundred Cloud Serpents"), are not as directly antagonistic as their nephews the Centzonhuitznahua, who attacked their mother in force and ended up the victims of a Huitzilopochtli-inflicted bloodbath. They are not directly violent, but they were said to have begun neglecting the necessary sacrifices to the sun and to have started getting drunk, being promiscuous and generally acting like socially unacceptable louts as far as Aztec society is concerned. Such disrespect and refusal to participate in the all-important running of the universe led Mixcoatl and his four closest siblings to take up weapons, ambush and kill all of them, whereupon they became the northern stars.

There are a few other anecdotal additions to the legend here and there - in one, the sun itself (and thus probably whichever god was in charge of it at the time, though the myth is uncertain about which world it occurs during) sends Mixcoatl after his brothers, while in another the sin that necessitates their death is that they attacked their mother Itzpapalotl (which is ironic, considering that after killing them Mixcoatl goes and fights her as well). In yet another, it's implied that Mixcoatl's own ambition played a large role, as he was not allowed to hunt large game since he was the youngest sibling, and removing his brothers meant that there was no longer any restriction on his power. There's also a sequel myth of sorts, in which some of the Centzonmimixcoa (which is not specified) return for revenge and manage to kill Mixcoatl, leaving Quetzalcoatl, his son, to take over rulership of the Toltecs.

Unlike the violent opposition of the Centzonhuitznahua, the problem with the Centzonmimixcoa is that they're refusing to be productive members of divine society; they're more comparable to someone like Tecciztecatl, who refused to sacrifice himself for the good of the world, which means that they weren't directly trying to murder anyone but are no less despicable in the eyes of a people who believed that their slacking off meant the whole world might blow up at any moment. Mixcoatl, who appears either sympathetic or ruthless depending upon what version of his motives you prefer, also comes out of the myth looking oddly ambiguous; are we glad he was taking charge to kick these failures' asses and restore order to the universe, or are we worried because he's on a fratricidal rampage in order to shore up his own power?

For the Aztecs, it's easy to come down on the side of worried, considering that he was the patron of the conquered Toltecs, aligned with the terrible realm of the stars and doing things that are suspiciously similar to what Huitzilopochtli, He-Who-Will-Not-Stand-For-Challenges-To-His-Authority, is usually doing. But Storytellers can always play with who was up to what and at fault; there are lots of options!