First of all, I want to thank MixMasterNut, his Nuketown loading screen analysis inspired this thread.
"Who is Mictlantecuhtli?"
Mictlantecuhtli (Nahuatl pronunciation: [mikt͡ɬaːnˈtekʷt͡ɬi], meaning "Lord of Mictlan"), in Aztec mythology, was a god of the dead and the king of Mictlan (Chicunauhmictlan), the lowest and northernmost section of the underworld. He was one of the principal gods of the Aztecs and was the most prominent of several gods and goddesses of death and the underworld. The worship of Mictlantecuhtli sometimes involved ritual cannibalism, with human flesh being consumed in and around the temple.
"Where is Mictlantecuhtli in Zombies?"
As you can see, and see better in MixMasterNut's thread, he is referenced in the Nuketown Zombies loading screen by a package of bubble gum. Once seeing him here, I was prompted to do more research.
In Mob of the Dead, the dead are in the Underworld of Mictlan, which is what he rules over.
"The dead had to pass many challenges, such as crossing a mountain range where the mountains crashed into each other, a field with wind that blew flesh-scraping knives, and a river of blood with fearsome jaguars."
Recognize the flesh scraping knives?
And then, the final piece of the puzzle, that put it all together for me.
As we all know, The Flesh are a cult which consume zombie flesh, and perhaps receive some sort of benefit from it, as they seem to be safe from the infected. "The worship of Mictlantecuhtli sometimes involved ritual cannibalism, with human flesh being consumed in and around the temple." Him being over all souls as well, explains how the MDT ran on souls, and why souls are so prominent in the story.
Mictlantcuhtli has been the voice in Richtofen's head the entire time! HE helped Richtofen to get in control, to cause an apocalypse, and create the Flesh cult, so worship of HIM could be held for all eternity.
I don't know about you guys, but I reaaaaally like this.
More info added by other users:
Day Itzcuintli (Dog, or the 6th day of the 365 day Aztec calender,) is governed by Mictlantecuhtli, God of Death, as its provider of tonalli (Shadow Soul) life energy. Itzcuintli is the guide for the dead, the spirit world's link with the living. Itzcuintli is a good day for funerals and wakes and remembering the dead. It is a good day for being trustworthy, a bad day for trusting others of questionable intent.
Looks like this can reflect on listening to Maxis/Richtofen, and Sal, Finn, and Billy listening to Weasel. They ended up dead, so the 6th day happened to be good for that. Apparently in Spanish, Mict is actually the same as Lucifer. Even more interesting, Jan 1st 1939 is the 6th day, or dog day, of the Aztec 365 day calendar. I think it's safe to say we know where we are in the Aztec world.