Slytherin are not all nazis – they’re a quarter of the population of wizards. It’s like saying ‘all Germans were nazis’ – just incorrect. Also, the chances are that some of the other houses have members that display nazi – like traits, but they are completely glossed over in the books and the films. After all, in both the books and the films, you meet very few of a year group, especially since hogwarts is the only wizarding school in Britain.