Basically, the digital brain is actually connected to her brain, and effectively a part of it. It's where she keeps some of her memories, and she can use it to connect to the internet in order to maunch global-scale cyberattacks. Once the hackers bomb that place with viruses (which happens because Neumann cannot keep using her talent to push back against it and overheats), it is completely destroyed. At that point, it has an effect on Neumann's physical brain, probably bursting a few veins or arteries. She starts bleeding, and has a cerebrovascular accident (stroke), leading to her death.
We see that all founding members except Kouu were present the night before the meeting when Touya was attacked by a sinner returner. Seiya probably already made up his mind at that point. Neumann asked Gale to control her between that night and the meeting the next day.