Well, I may be off on this, but I've always felt that being put in that kind of a situation (instantly becoming a billionaire, etc.) is going to
reveal someone's true character rather than fundamentally change that person's character.
But honestly, I feel like that still kinda misses the point. We're not talking about someone getting a million dollars and being selfish. We're talking about someone gaining massive amounts of power and
suddenly becoming a mass-murdering megalomaniac who wants to control the entire galaxy. I'm sorry, but people don't just drastically change like that without being a bit unhinged in the first place. Especially not Cortana, who always showed a lot of care for the Master Chief and even sadness for the marines that died.
Ok, obviously none of us are galaxy-spanning AIs so who's to say what how we would react in her situation, but this is a
story 343 is trying to tell her. If you're going to write a character-focused story that involves one of the main characters suddenly becoming the antagonist, and you want it to impact the audience emotionally, then that character change should make sense. You should be able to understand and at least sympathize, if not necessarily empathize. Cortana's change was like flipping a switch, without any explanation for the she would suddenly go full "ends justify the means".
"You don't know how you would react" is a really weak justification for poor character development, IMO.
Halsey would, which is why Cortana would, and Halsey knows this, thus Halo 5.
So I'm getting the impression that this is all connected to 343's portrayal of Halsey, and therefore leaking into their portrayal of Cortana?