They're just books, and while there does seem to be a "Slytherins are evil!" feel, remember that their core traits aren't necessarily evil: ambitious, resourceful, adaptable, cunning. I'm part of a Harry Potter community and the Slytherins are awesome (as are the Gryffindors and Ravenclaws and Hufflepuffs). While it's great that this inspired you to want to be a better person, don't put so much stock in it. Be proud of the good things you do.

Also, the Pottermore test is bullshit. Seriously.