Why didn't GLaDOS kill Chell?
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GLaDOS said to Chell (Portal’s player character) that killing her is hard. GLaDOS is supposedly more powerful than Chell, and she had the chance to kill Chell so many times, like:
She could've killed her from the beginning.
She could've crushed her alongside Wheatley.
She could've left her on the moon.
She could've used the turrets in the ending instead of making them sing.
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She could've killed her from the beginning.
In the beginning, Chell was just another test subject. GLaDOS had no reason to kill her yet. In fact she needed her alive to perform portal gun tests.
Yes, there are tests which contain deadly obstacles. But those are parts of the tests. Chell's death would not have been intentionally caused by GLaDOS. It would have been a side-effect of the test setup.
GLaDOS antagonism doesn't really start until Chell refuses to get incinerated at what was supposed to be her last test and starts to explore the backrooms of the Aperture Science facility.
Also note that GLaDOS does not actively do anything to kill Chell until just before the final boss fight, when Chell incinerates her morality core. It was installed in her to "make her stop flooding the enrichment center with a deadly neurotoxin", which she then immediately starts doing. So it is reasonable to assume that her morality core actually prevents her from intentionally killing someone unless it's part of an official test protocol.
Which means she was actually prohibited from actively killing Chell until that point.
She could've crushed her alongside Wheatley.
At this point GLaDOS doesn't have a morality core anymore, and has a good motive to kill Chell - as revenge for murdering her at the end of Portal 1.
But looking at https://youtu.be/7GXanBcXnso?t=190 , it seems like GLaDOS doesn't want it to be a fast death. She wants Chell to suffer. Which is why she throws her down into another series of increasingly sadistic tests.
She could've left her on the moon.
She could've used the turrets in the ending instead of making them sing.
At this point of the story, GLaDOS is no longer the antagonist. Wheatley is. She went through a lot of character development during the events of Portal 2 and more or less made peace with Chell. She still doesn't like her, but she is at least not so angry anymore that she wants her dead. She " https://www.youtube.com/watch?v=dVVZaZ8yO6o ".
