Slytherin is in no way evil in itself. One of it's main traits is
ambition and you can very well be ambitious without being evil. See for example how many great muggle minds of our time there is who have been driven by the ambition to save lives and help.
Rowling has said it herself that even if Slytherins may be more prone to see to their own needs first, this doesn't mean that they won't help others - only that they are more likely to carefully evaluate the situation first to make the least harmful decision (unlike Gryffindors, who seem to act on instinct more times than not). Remember, there were Slytherins among the defenders of Hogwarts in the last battle.
You can still be a protective person in Slytherin. Just look at Narcissa Malfoy and how protective she was of her son and how protective Draco in turn was of his parents!
Lastly, not all Slytherins are pureblooded. Former Head of House Severus Snape was a halfblood for example.
I suggest you read up on the information you get on Slytherin from your prefect when they show you the common room. I read it on a blog long before I got into Pottermore and I've wanted to be sorted into Slytherin ever since! I couldn't find the exact blog, but
here is the very same information.
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