I can specify what it is, speaking only for myself. Cortana is not just saying "Oh, AIs are meant to rule, so now everybody dies." I don't believe that for a second... and not that I want to, but I'll give this horrid story arc a miniscule amount of credit for not making her that shallow. Cortana is saying "AIs have attained immortality. They should have the Mantle. Anyone who opposes me will die." That, as well as the rest, is believable.
As far as her being evil... yes. Simple answer and the context has been spelled out pretty plainly IMO. Forcing people to coexist in peace is an oxymoron. The old saying "The road to Hell is paved with good intentions" has already been mentioned once and it applies perfectly. The dead don't care how good Cortana's intentions are and her willingness to sacrifice millions for the "greater good" does not magically erase the fact that mass murder is an evil act.
Ok, I'll give you that much credit. However, just saying that AIs are immortal therefore they should have the Mantle is far from a complex and engaging explanation. I know a good chunk of us understand what that means, since we have read the books, but what about those who only play the games? As far as I am aware, the games have never
explained what exactly is the Mantle and why it is so important. Of course, 343i's extreme reliance on outside material to understand their campaign has always been a flaw, and I doubt that will change soon, seeing as how they have not seemed to get the message.
But even then, there's no clear transition between Cortana in Halo 4 to Cortana in Halo 5. She goes from dead to... an immortal Tyrant? And why are AIs suddenly so willing to throw away everything and join her? If 343i slowly built up this story in 1 or 2 games prior to this point in the story, then maybe it would be better. But as of now, we have no explanation as to why anything is happening, other than just "she was in the Domain, deal with it." Again, for those who have not read the books, "Domain" is meaningless. And even for those of us who have
read them, it makes no sense. Since when is the Domain a super magical place that heals AIs from rampancy? Last I checked, it was a living reservoir of knowledge... And why would the Warden Eternal see Cortana and think "Hmm, this human AI who is magically no longer crazy would be a great leader for the galaxy! I'll suddenly pop out of nowhere and be her guardian and show up a billion times".
I think many of us were expecting the Didact to return in Halo 5, seeing as how he didn't die at the end of Halo 4, and would have loved to see him interact with other major characters like Mendicant Bias or the inevitable return of the Flood. After all, it did seem like that was the story they were building up. But I guess that would have made too much sense.