JK Rowling did a terrible job of writing Slytherin house and its students. if anyone wants to read my rant of reasons here it is:
a) why does Slytherin ambition always have to be for power? for some people they might be willing to do bad things for power, true, but that really isn't the case with a lot of people. maybe I just have an ambition to clean my room, or eat an entire bin of ice cream.
b) how come the slytherin common room is so musty and dirty? does anyone really thing that Lucius Malfoy would let his son spend the school year living in those conditions? If anything Draco would definitely "tell my father about this" ;)
c) think of a Hogwarts student in Slytherin right now. any one of them. I bet most of you said Draco Malfoy, right? yes, he was kind of a jerk. JK Rowling said he was based of of a school bully, that makes sense. but don't make that character the shining example for something that you don't need to give a bad reputation. even though there were some good Slytherin characters in the books, they were few and it is much harder to think of good Slytherins than bad Slytherins.
d) the main characters in the books show a blatant favoritism towards Gryffindor, and hate Slytherin. honey, no. ALL. THE. HOUSES. ARE. EQUAL. the houses are really only as good as the students in it.
thank you for coming to my ted talk. feel free to add on or rant about something else harry potter related :)