Slytherin was founded by a murderous bigot and for centuries has been a symbol of pure blood nonsense. In modern times, its alumni included magic Hitler, most members of the magical KKK, and a man who sold out his childhood crush to magic Hitler because she chose someone else. During the Battle of Hogwarts, the Slytherin students were sent to the dungeon because so many of them had parents in the Death Eaters and couldn't be trusted.
The least flawed Slytherin character in the books, Slughorn, is obsessed with forming connections with the magical world's wealthy and famous. Still not someone you'd probably want as a friend.
Haven't seen the Cursed Child, so maybe it actually presents a more positive picture of Slytherin. But the original books certainly don't.
