Most mythological monsters want something. Food, territory, revenge.
Xecotcovach wants nothing. It's an executioner with feathers — sent, not hungry. And that makes it far more terrifying than anything that simply wants to eat you.
The Mayans built this bird into their creation myth as a correction. An earlier draft of humanity failed. These "wooden people" could walk and speak but were hollow inside. Soulless in a way the world couldn't tolerate.