Granted, Carrie White probably shouldn't have telekinetically murdered her high school classmates at the prom. But her home life almost justifies it. Mrs. White — played in the movie by Piper Laurie — is a religious maniac who believes that Carrie is getting her period because she is sinful, and that the proper remedy, rather than a tampon and an awkward conversation, is to lock her in a closet and make her pray. Also, she refers to her daughter's budding breasts as "dirty pillows," which is just bizarre.
Laurie reportedly thought her character was so over-the-top that the movie she was acting in must be a comedy. Which of course didn't turn out to be the case, but you can see where she was coming from: Margaret White's religious mania is beyond caricature — "Eve was weak! Say it, woman!" Though in her defense, it's worth pointing out that on some basic level Carrie's mom was right: Carrie was a witch, and it probably would've been better if she hadn't gone to the prom. If she'd only been a little quicker with that kitchen knife, so much unpleasantness could have been avoided. Kids: what they really want is discipline.