After some time to mull it over, I came to a very different conclusion surrounding Yotsuyu's actions during 4.3. It's still very tragic - perhaps moreso than your interpretation - but a bit more hopeful, I think.
Yotsuyu never really came back. The "Yotsuyu" we see is, for the most part, Tsuyu playing Yotsuyu.
We know she regained her memories in Namai and when confronted with her parents. After delivering the persimmon Gosetsu wanted to him, she nearly commits suicide out of guilt for her past actions and feeling unworthy of Gosetsu and the others' kindness and mercy. She's then stopped by her parents, who continue to treat her like a burden and piece of property, only to kill them in a rage. Afterwards Yotsuyu plays out Asahi's assigned role in the tragedy until the very end, using the last of her strength to defy and kill him.
However... "Yotsuyu" seems to be trying too hard to play the role of the iron-heeled Witch of Doma. Before incarnating herself into Tsukuyomi she grandstands at length about how she's an Imperial citizen, and how she's going to make Doma suffer and pay both to fulfill her duty and for personal satisfaction. However, her actions as Tsukuyomi don't really support that being her true intent. Instead of attacking right away she waits patiently for the others to evacuate and for the Warrior of Light to charge her. After just a few moments of combat, she's so weakened that she requires external stimuli in the form of phantoms to power herself up, and even when trying to do that is stopped short by a phantom of Gosetsu holding off Zenos. After using her Nightbloom super, she seems to be fully aware and accepting of the fact the Warrior of Light is going to kill her - she feels she's beyond redemption, despite Gosetsu's protestations to the contrary. In the end she finishes off Asahi, letting cooler (and saner) heads prevail.
Yotsuyu knew everything that would happen, and she played it all to an ultimately good end she sacrificed her life for. She knew Asahi's plan (obviously), and she knew the Warrior of Light would be there. She knew that playing "Yotsuyu," claiming Imperial citizenship, would place the onus for the summoning on the Empire, and she knew (incarnate) summoning would force the Warrior of Light to kill her. She also knew Asahi would twist events on paper, claiming a Doman citizen was responsible, so saved the last of her strength to kill him and allow a clear version of the events to be relayed back to Garlemald - not only sparing Doma yet another bloody war, but exposing the nefarious elements of the Empire by sending the genuinely populist Garleans back with that correct record of events.
In short, Yotsuyu (or Tsuyu) wanted us to kill her because she didn't feel like she deserved the kindness and mercy Gosetsu and everyone else offered her after what she did, and knew that summoning would force the Warrior of Light to kill her. She played that to her advantage by declaring Imperial allegiance and saving the last of her strength to cut the head off the serpent running the game (though personal satisfaction was also a factor), ending the life she felt she didn't deserve to do it. Even with her memories returned Tsuyu had become her true persona, while "Yotsuyu" was just a mask she wore to deceive everyone into playing the parts she wanted them to. (Ironically the exact opposite of what so many were suspicious of.)
Perhaps. Or perhaps it's just my ego and naivete talking.
Either way, the Warrior of Light is clearly not happy with how things turned out and enraged by Asahi's barbarism, so I do not agree with the conclusion that they are little more than a killing machine seeking thrills without scruples.