Meg looked. The dark shadow was still there. It had not lessened or dispersed with the coming of night. And where the shadow was the stars were not visible.
What could there be about a shadow that was so terrible that she knew that there had never been before or ever would be again, anything that would chill her with a fear that was beyond shuddering, beyond crying or screaming, beyond the possibility of comfort? (4.131-132)
The focus on time here is interesting – the Black Thing is not just the worst thing at this moment, but the worst thing that every was or ever will be. It's interesting too that the text focuses on Meg. On the one hand, it makes sense, since she's our window into the text (see "Narrator Point of View"), but since the text has also emphasized the limitations on her understanding, focusing on the Black Thing being the worst thing she could experience doesn't close off the option of there being something so bad it's beyond Meg's ability to comprehend it.
Calvin turned, rejecting the dark Thing that blotted out the light of the stars. "Make it go away, Mrs. Whatsit," he whispered. "Make it go away. It's evil." (4.136)
Calvin's reaction seems almost visceral – without any knowledge of what the Black Thing is or does, he immediately judges it to be evil. Perhaps evil is beyond reason?
Again Mrs. Which's voice reverberated through the cave. "Therre willl nno llonggerr bee sso manyy pplleasanntt thinggss too llookk att iff rressponssible ppeoplle ddo nnott ddoo ssomethingg abboutt tliee unnppleassanntt oness." (5.85)
Mrs. Which points out that there's no opting out of the good vs. evil smackdown, because everyone will be affected by the outcome.
"It's the Thing!" Charles Wallace cried. "It's the Dark Thing we saw from the mountain peak on Uriel when we were riding on Mrs. Whatsit's back!"
"Did it just come?" Meg asked in agony, unable to take her eyes from the sickness of the shadow which darkened the beauty of the earth. "Did it just come while we've been gone?"
Mrs. Which's voice seemed very tired. "Ttell herr," she said to Mrs. Whatsit.
Mrs. Whatsit sighed. "No, Meg. It hasn't just come. It has been there for a great many years. That is why your planet is such a troubled one." (5.100-103)
In a way this harkens over to the fate vs. free will debate happening next door to good vs. evil – if people do bad things because there's a black cloud over their planet, are they really responsible? What role does choice play in how the Black Thing influences a society?
"But what is it?" Calvin demanded. "We know that it's evil, but what is it?"
"Yyouu hhave ssaidd itt!" Mrs. Which's voice rang out. "Itt iss Eevill. Itt iss thee Ppowers of Ddarrkknesss!" (5.108-109)
Mrs. Which's answer is circular: it's evil 'cause, well, you know, it's evil.
Suddenly there was a great burst of light through the Darkness. The light spread out and where it touched the Darkness the Darkness disappeared. The light spread until the patch of Dark Thing had vanished, and there was only a gentle shining, and through the shining came the stars, clear and pure. Then, slowly, the shining dwindled until it, too, was gone, and there was nothing but stars and starlight. No shadows. No fear. Only the stars and the clear darkness of space, quite different from the fearful darkness of the Thing. (6.4)
It's interesting that the defeat of the Black Thing doesn't lead to the universe being lit up like a baseball stadium, but rather to an absence of unnatural darkness. It's almost like the battle isn't so much between evil and good as between evil and the normal or the natural.
Without warning Meg was swept into nothingness again. This time the nothingness was interrupted by a feeling of clammy coldness such as she had never felt before. The coldness deepened and swirled all about her and through her, and was filled with a new and strange kind of darkness that was a completely tangible thing, a thing that wanted to eat and digest her like some enormous malignant beast of prey. (6.68)
The digestion metaphor foreshadows what's going to happen on Camazotz – IT doesn't want to destroy the people around IT, but to incorporate them into itself. Why?
She teetered on the see-saw of love and hate, and the Black Thing pushed her down into hate. "You don't even know where we are!" she cried out at her father. "Well never see Mother or the twins again! We don't know where earth is! Or even where Camazotz is! We're lost out in space! What are you going to do!" She did not realize that she was as much in the power of the Black Thing as Charles Wallace. (10.67)
Meg's experience suggests that evil is not an all-or-nothing proposition – even though she didn't give in to IT, she's still touched by IT's evil. While on the one hand the materiality of the Black Thing as an entity that is located in specific places seems to divide the world into good and evil, Meg's crankiness suggests that matters are not so simple.
I hope I don't smell awful to it, she thought. But then she knew with a deep sense of comfort that even if she did smell awful the beasts would forgive her. As the tall figure cradled her she could feel the frigid stiffness of her body relaxing against it. This bliss could not come to her from a thing like IT. IT could only give pain, never relieve it. The beasts must be good. They had to be good. She sighed deeply, like a very small child, and suddenly she was asleep. (11.38)
Meg's morality at this moment is based in her senses – pain bad, pleasure good. And, in these circumstances, she's right. What else in the book supports or denies this approach to morality?
Aunt Beast spoke to the others. "The child is distraught. Don't judge her harshly. She was almost taken by the Black Thing. Sometimes we can't know what spiritual damage it leaves even when physical recovery is complete." (11.104)
It's like not only Meg's body, but her "spirit" are the battleground for this fight between good and evil – even when there's not active fighting going on, the damage remains.