There are already answers in here explaining all of this but one thought struck me as I was considering why Salazar Slytherin, blood purist racist, would hide a basilisk in a contraption designed by the very people he hates?

My conclusion is that it is the perfect insult and makes for a great cover up. Slytherin, being a powerful and prestigious wizard, would never be expected to engage in muggle artifacts and so by hiding a muggle-born killing weapon in a space familiar to muggle-borns would like be laying out a honey trap and then back-hand smacking the victim once they "get trapped".

There's also plausible deniability since why would the heir of a powerful pureblood be opening a muggle-contraption to find an entrance to a pureblood wankchamber?

It's sort of like punishing Indiana Jones with a whip (dunno why the fuck this was the only thing I could think of) - but it would be adding insult to injury.