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No, she didn't know what happened at the prom. She tried to scare Carrie into not going to the prom by telling her they will all laugh at her because she remembered how Carrie told her how the girls laughed at her when she was panicking over her period in the shower, so she tried to play on that fear, but she had no idea they would actually humiliate Carrie and laugh at her.
And yes, she planned to kill Carrie no matter what because she realized she had lost control of her and believed she was a witch who was under the control of satan. So, even if Carrie had enjoyed a marvelous time at the prom, her mother was still waiting to kill her when she got home.
This is a faithful sayingJesus came into the world to save sinners; of whom I am chief.

I don't think mother knows what happened. I think all Carrie says is they all laughed at me. And yes, she planned on killing her regardless.

The quote was
Thou Shalt Not Suffer a Witch to Live
.
There is no indication that the mom knew anything like THAT would happen, and she certainly had no idea of what Carrie would do with her powers.
The mom told her that the classmates would laugh at her because
Margaret knew her daughter was shunned, because she WORKED TO KEEP HER THAT WAY. Margaret used Carrie so she herself would not be alone. It wasn't only about so-called sin, but her own fear that Carrie would leave and have her own life.
She tried desperately to keep Carrie all to herself.
I'd say this cloud is Cumulo Nimbus.
Didn't he discover America?
Penfold, shush.

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