Why did Salazar Slytherin arm the Chamber of Secrets with a Basilisk?

Question

Why did Salazar Slytherin arm the Chamber of Secrets with a Basilisk? If the main reason was, as the legend says, to "let his heir purge those who were unworthy to study magic" (i.e., kill Muggles) it seems woefully inefficient and needlessly complicated. There had to be a better way than that! Also, there is a great risk of Wizard children dying as well - as they are not immune to the basilisk!

All in all, there must be a better, simpler way to purge the school of Muggle children. So why did Salazar Slytherin do it that way?

(I have my own theory - see it here: https://scifi.stackexchange.com/a/220284/89771. But it's merely a hypothesis. I want to know the reason according to current canon - i.e., that he made the chamber so his heir could kill Muggles.)

Answer 1

Salazar’s reason is never truly confirmed.

It isn’t known for certain why Salazar Slytherin put the basilisk in the Chamber of Secrets. Though the often-cited explanation is that he did it so he or one of his descendants could rid the school of all those who were unworthy of studying magic, it’s unclear if this is proven anywhere or if it’s just a persistent rumor.

“The story goes that Slytherin had built a hidden chamber in the castle, of which the other founders knew nothing.

‘Slytherin, according to the legend, sealed the Chamber of Secrets so that none would be able to open it until his own true heir arrived at the school. The heir alone would be able to unseal the Chamber of Secrets, unleash the horror within, and use it to purge the school of all who were unworthy to study magic.”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 9 (The Writing on the Wall)}}

Many people believed this, including the Dark Lord himself, who thought that he would finish what his ancestor started by opening the Chamber and using the basilisk to kill Mudbloods.

“I decided to leave behind a diary, preserving my sixteen-year-old self in its pages, so that one day, with luck, I would be able to lead another in my footsteps, and finish Salazar Slytherin’s noble work.”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 17 (The Heir of Slytherin)}}

However, whether this truly was his plan is never actually confirmed. Professor Binns told his class that reliable historical sources said that Salazar disliked taking students born to Muggles because he believed them to be untrustworthy, and he fought with the others about it. However, he further explained that even the existence of a Chamber wasn’t in these sources. That means that Salazar’s reason for putting a basilisk in it also wouldn’t be recorded in any reliable sources, and therefore it’s not certain what it had been.

“He paused, gazed blearily around the room, and continued, ‘For a few years, the founders worked in harmony together, seeking out youngsters who showed signs of magic and bringing them to the castle to be educated. But then disagreements sprang up between them. A rift began to grow between Slytherin and the others. Slytherin wished to be more selective about the students admitted to Hogwarts. He believed that magical learning should be kept within all-magic families. He disliked taking students of Muggle parentage, believing them to be untrustworthy. After a while, there was a serious argument on the subject between Slytherin and Gryffindor, and Slytherin left the school.’

Professor Binns paused again, pursing his lips, looking like a wrinkled old tortoise. ‘Reliable historical sources tell us this much,’ he said, ‘but these honest facts have been obscured by the fanciful legend of the Chamber of Secrets.”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 9 (The Writing on the Wall)}}

While the existence of the Chamber was proven to be true afterward, part of the story being true doesn’t prove that all of its pieces are true, and there are no other known sources stating why he chose to put a basilisk in the Chamber. Therefore, there’s no way to be certain with the information we have what the true reason why Salazar Slytherin put the basilisk in Hogwarts is.

Answer 2

tldr: he used a basilisk because that was the best option available to him

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This answer is based on the assumption that all the information we have about Slytherin are correct. I also recommend you to read and upvote this as it's a good answer. But I also think it's just a theory, and as more new material gets added to the Harry Potter universe, I think we will be able to come up with more and more fan explanations to fill up plot holes in the main books, regardless of whether they might be correct or not. Also, the books don't imply that any thing the readers get to find out about Slytherin are false, so this answer is based on, as Prof. Binns put it,

...solid, believable, verifiable fact!"

Prof. Binns says that

"Slytherin, according to the legend, sealed the Chamber of Secrets so that none would be able to open it until his own true heir arrived at the school. The heir alone would be able to unseal the Chamber of Secrets, unleash the horror within, and use it to purge the school of all who were unworthy to study magic."

He also goes on to say

"That is believed to be some sort of monster, which the Heir of Slytherin alone can control," said Professor Binns in his dry, reedy voice

So he needed something that
- could be used as a weapon to get rid of people unworthy to study magic
- could be controlled by his true heir
- could be stored without being detected while he is not around

Given all these conditions, the basilisk is the perfect weapon for him, as
- it turned out it survived for more than 1000 years, staying dormant until called out
- it could travel through the castle undetected
- it did everything it was told to by his true heir

Also, if we are to believe Voldemort inherited his narcissism from his great great...grandfather, then a magical giant snake is a perfect thing to be placed there, as it was the symbol of his house, as well as the thing he was famous for being able to talk with. Voldemort is shown to put a lot of importance on symbolism that exemplifies his greatness, and his link to magic and Slytherin. Perhaps this trait comes to him from Slytherin himself, in which case it is more logical that Slytherin would decide to place a snake there.

Also, as @TheAsh has pointed out at other places, "get rid of people unworthy to study magic" may or may not mean killing muggle born students at the school. But whatever that implies, he needed to hide something dangerous in the chamber, which would also be subservient to his heir. As the events in the book showed, the basilisk was perfect for this.