Everyone talks a lot about how Autloc's faith is destroyed in this story, but I think it's interesting to look at the depths Tlotoxl is driven to in "The Day of Darkness". He started the story as a keeper of the faith and defender of the Aztec way of life--if you don't mind me getting culturally relativist for a moment and pretending that a society built on imperialism and the brutal sacrifice of one's enemies to appease a bloodthirsty god isn't messed up. The point is, he is meant to represent a true believer in the Aztec society's code, and his objection to Barbara is that she's a con artist trying to lure them into heresy and the abandonment of their gods.
But in his desperation to drive Barbara and everything she represents out of his society, he's driven by this episode to manipulate the leader of the Aztec warriors into braining a high priest from behind. Barbara's schemes have driven to become the very thing he's terrified of, and the brilliance of his final scenes is that he knows it. His spitting, snarled epithet of "Let her go!" is the choked frustration of a man who feels defeated even in victory, and there's something forced about his invocation of sacrifice that belies the scene's attempts to convince us that he's won.
Everyone's desperate in this episode; after a brief moment of false hope, when Ian rigs the door at the beginning, there's a very real sense for much of the rest of the episode that the Aztec civilization is slowly closing a trap on them. Ian and Susan are framed for assault, Barbara's about to be entombed alive, and the Doctor...even though his fate seems like it's to be more pleasant than most, he's nonetheless trapped in this culture's ethos and ideals. Obviously, the trap is evaded, but the episode functions perfectly as a climax to the story. Having chosen to join the Aztec society in order to transform it, Barbara discovers almost too late that the role she's chosen is a prison without bars.
They do escape, of course, but not before Ian and Ixta have their final battle. This is a little bit of a strange note to strike; Ian is, after all, a maths teacher by trade. His scenes leading up to the battle, where he seems to be almost eager to have things out with Ixta once and for all, are surprisingly stone-cold and tinged with machismo. And yet, this is as much a watershed moment for Ian as last episode's confrontation with Tlotoxl was for Barbara. The two schoolteachers are coming around to the idea that this isn't Earth, and that they'll have to be very different people in order to survive their travels with the TARDIS. This story has left its mark on them, even if they didn't make a mark on history. That's the reason why the historicals always worked so well, even though they could "never affect things"...they could be affected, which is always the far more interesting outcome anyway.