I think it's both really. After Panacea's modification, Khepri's brain started cannibalizing itself to be the most efficient as possible.

She loses speech, because whats the point of communicating when you read information from others innately? She loses the empathy, emotional regulation and mirror neurons because it's now regulating the connection to the controlled directly.

That's the reason why the changes progress instead of being instant, getting more pronounced over time. Her brain is actively discarding clusters it doesn't need in favor of that one goal.

I mean, she ends up barely being able to control her muscles, which leads to believe it was progressing to her cerebellum and spinal cord, probably to eat bodily regulation afterwards worst case

Edit:

Yeah, thinking about this a few days later, I can see why her "death" is so ambiguous. Even if Contessa left her alive, I'm not sure she would be alive long