According to albus dumbledore, salazar slytherin's qualities in selecting students are intelligence, resourcefulness, strong will and "some contempt for laws and regulations". Dumbledore pointed out that Harry Potter has all these qualities, but he is at Gryffindor College.
On the contrary, Vincent Crabbe and Gregory Goyle were all stupid and were assigned to Slytherin. The former is because Harry Potter strongly urges not to enter Slytherin, while the latter may be that they have some secondary Slytherin characteristics, just as peter pettigrew entered Gryffindor obviously not because of bravery, but because of chivalry. ?
Extended data:
In the series of Harry Potter novels, J.K. Rowling seems to describe the wizard from Slytherin as incompatible with other upright wizards: at Hogwarts, Slytherin is always pretentious and arrogant; Strive for victory by hook or by crook and do your best; Derogating the other three colleges is also disliked by the other three colleges.
But at the end of the seventh book, nineteen years later, Harry told his son that evil is a wizard, not a college. He felt that even if his son Albus Severus Potter got into Slytherin, there would be no problem.
People are always willing to generalize and make a face of the four colleges-Gryffindor, Slytherin, Ravenclaw, and Hufflepuff. But in fact, Gryffindor has peter pettigrew and Dumbledore, and Slytherin has Voldemort and Snape.
Slytherin is not evil. Like the other three colleges, it's just a group of children. Just because of the different education, these children are more likely to grow up to be unscrupulous people.