When you think about it. The series is really depressing but also heartwarming. GLaDOS was Caroline. Cave Johnson and the lab boys forced her to become GLaDOS. To take away her freedom and move her into a computer. No one would ever know it was her. She flooded the Enrichment Center with neurotoxin to kill the people who did it to her out of rage. Later, she felt bad about it but what was done was done. So she developed a mindset that, like the people who basically killed Caroline. Everyone was horrible. She eventually distanced herself from the nice, cheerful woman who Caroline was because she wasn't Caroline. She was a murderer. She awoke Chell, and very quickly begins to show distaste for humans. Throughout Portal 2, she insults you on weight, lack of parents, and personality. Just because she doesn't have that. And she killed hundreds of people, and took it out on you. At the end when she says "You were my best friend." She means that as in, you're her only friend. You're the only person