SPOILERS

So everyone knows that Cave Johnson was dying due to his experimentation with using moon rocks to create conversion gel for the portal gun. This led to the AI technology research that had Cave convert Caroline into the AI GLADoS. We always had assumed that GLADoS kills the staff of Aperature as revenge for her conversion into an AI, but I think it might have been something else.

The massacre of the Aperture staff happens on Bring Your Daughter to Work Day, and we have always assumed that Chell is the daughter of Cave and Caroline. Now at this point, Cave is presumably desperate, dying, as well as already have turned his lover into a artificial intelligence. The first attempt to digitize a human mind didn't work completely, as there are bugs being worked as of the Bring Your Daughter to Work Day, as shown by Rattmann's account. So Cave would presumably try again, and I think Chell is the one Cave tried it on. I think this is what sent GLADoS into her homicidal rage.