I think that, ultimately, the fact that Aztechnology provoked the dragons with the execution/sacrifice of a dragon can't really be called the reason for Sirrurg's actions, but more accurately described as the excuse.
The one that the Sixth World calls Sirrurg was known in the Fourth Age as Usun, and to the dragons in both eras as Vast Green. Vast Green is a dragon supremacist. He believes that the dragons should be ruling the other races, or else exterminate them. He's wanted nothing more for the last ten thousand years than to raze (meta)human civilization to the ground and preside over the ashes, but this was forbidden by the Dragon Council, so barring that he'll settle for killing any that he can get away with killing and he's not in the least particular about who.
In the Fourth Age, having been forbidden by the decision of the dragon majority from acting on his desire for dragon domination and being loyal enough to dragonkind to obey it, he hid away in a jungle and ate anyone who came near him. He only took interest in outside events when it seemed that there might be a war in the offing, so he could take the opportunity to attack metahumans in a sanctioned action. In the Sixth World, the authority of the Council seems to have waned somewhat, and he's slipped his leash.
When Sirrurg was helping Hualpa and the Amazonians take over Brazil, there were frequent complaints about him attacking friendly forces and civilians, apparently just because he felt like it. One suspects that he was more interested in the excuse to attack a human nation than in doing Hualpa a favor. When Sirrurg attacked that flight, yes he was hunting one of the downcycle hunters, but he chose to attack at the location where it was all but certain to cause massive loss of innocent life... where, in fact, the one he had set out to kill was the only one with a chance to survive! (If a plane is going down, a mage can use Levitate to escape. Everyone else is screwed.) Now in Aztlan he kills an entire population of unarmed civilians with a sadistic death spell, apparently for no other reason than because he can.
Sirrurg lives by the law of the jungle. Heck, to the natives of the Liaj Jungle where he used to lair, he was the jungle made manifest: uncaring and capricious death, devoid of any delusions of morality or mercy, allowing only the strong to survive. Sirrurg might be currently aimed at Aztlan because of this latest offense, or their use of blood magic, or some tie to ancient Thera. But what he does is what he's wanted to do to all of us since the distant past: wipe us off the face of the Earth. And I'll bet anything that he's having fun.
