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I don't believe it's been explicitly stated. I have three guesses though:

1. Caroline was thought to be against the process of having her consciousness transferred to GLaDOS as evidenced by some cut sound files. So maybe that part of Caroline deep down probably wanted revenge, and in the fully activated AI manifested in GLaDOS's urge to murder.

2. The story "I Have No Mouth and I Must Scream" is about a murderous AI similar to GLaDOS (In the fact that it is a murderous AI) named AM. This AI went insane due to the merging of sentience with machine. While it had all of the feelings and longings of a living person, it's ability to fulfill its desires were crippled by its mechanical programming. So it blamed humanity for this and went on a massive rampage.

3. Similar to HAL 9000 in 2001:A Space Odyssey, maybe GLaDOS felt threatened by her masters. She was completely sentient and intelligent, but she was under the control of programmers and technicians who could shut her down at the flip of a switch, and so she found some way to take control and preserve herself.(Actually HAL was not sentient and was affected by the paradox of never keeping anything from the crew, and not being allowed to let the full mission objective be known.)

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