People were confused why Cortana has become a villain,and her motivations for doing so. People theorize that she is being controlled by the Didact (which could be true), but there is another theory, which many subscribe to. This theory is that Cortana is suffering from logic plague, seeded by the Gravemind/Primordial. Logic plague is the techniques used by the Flood to attack and subvert intelligences through non-biological means. We have several examples of beings as victims of logic plague:
- Mendicant Bias: The contender-class AI conversed with the Primordial for 43 years on Installation 07. The Primordial convinced Bias that the Forerunners had twisted the meaning of the Mantle as the Precursors intended, and that the Flood would elevate the galaxy to a higher state of being, free of conflict. The Primordial told Bias that due to the Forerunners’ resistance to the Flood, they were holding the galaxy back from this excellence. This caused Bias to develop a deep hatred for the Forerunners, and turned against them. - Ur-Didact: The Ur-Didact was left in a burn and was brought to the Primordial/Gravemind, in order to cause disorder in the Ecumene. In the clutches of the Primordial/Gravemind, many of the Didact’s personal views became more extreme. Beforehand, he didn’t like Humanity but had a certain respect, now he flat out despises Humanity. Originally, he was opposed to the Array as it was against the Mantle, now he holds disgust for the Array. In the past, he believed the Forerunners were the rightful inheritors to the Mantle and guide the galaxy, now he believes the Forerunners are the dominant species, and it’s their right to rule the galaxy. The Primordial/Gravemind then sent the Didact back to Forerunner space, now suffering from logic plague, and caused distress the Faber, the Librarian, and Iso-Didact in his change in views. - Cortana: Now we get to the big one, Cortana. This dates back all the way to Evolutions: Human Weakness, where it’s the story about what happened between Cortana and the Primordial/Gravemind after John left High Charity. The Primordial/Gravemind conversed with Cortana, with him saying stuff like her being enslaved to inferior organics, with Halsey designing her to die, and that John wouldn’t come to save her. Cortana at first tried to blow him off, but eventually began to grow fearful of him, and most likely started to feel many of those things he was telling her. In Halo 4, when John and Cortana approach Ivanoff Station, Cortana says “Do you know what that condescending -Yoink- said to me after our first game of chess?!” This was her rampant side coming out yes, but what if, deep down, this is due to the logic plague?In Guardians, Cortana has such dislike for Halsey, calling her a monster for what she did to the Spartan IIs. She believes AIs are more fit to be the ones to hold the Mantle, as supposedly the Domain/Organon and can have them live forever. She believes very similarly to the Didact, pretty much quoting him verbatim Didact: “The Mantle of Responsibility for the galaxy shelters all Human. But only the Forerunners are its masters.” Cortana: “The Mantle of Responsibility for the galaxy shelters all, but only the Created are its masters.” Due to the quotes of the two pretty much being identical to each other, there are those who believe that Cortana is either secretly the Didact, or being controlled by him. While this may turn out to be true I don’t think it’s the case. I believe the Primordial/Gravemind planted the seeds inside Cortana, which he know would consume her eventually. I think Cortana heard Didact say those things, and when she connected to the Domain/Organon, she saw the opportunity to spread her real views. In a way, Cortana’s logic plague is mixture of Mendicant Bias’ and the Didact’s, she has turned against her creators, believing they aren’t correct, and causing problems, and turns against like, like Mendicant Bias. Like the Didact, she believes her people, (AIs) are the dominant people in the galaxy, the only ones worthy to hold the Mantle, and everyone must listen to them. This is just a theory, and would be interesting if it turns out to be true.