You had been in testing with Chell since the beginning, with GLaDOS commenting throughout it. Then, you finally were at the end. Chell said that GLaDOS promised cake, and there she was. Chell, who was tasked to destroy GLaDOS, took out a personality core(she thinks). But, to both of your dismay, it was her mortality personality core.

"That was my mortality core, the scientists put it in when I flooded the lab with neurotoxin," GLaDOS begins to explain. "And now I have no limits to stop me from doing it again."

Suddenly, the room began to fill with green gas, which you assumed was neurotoxin, with timers counting down by the millisecond on all sides of the walls. Chell looked at GLaDOS with a determined expression, like how two rivals would look at each other.