Quoting Blair Waldorf (SuperKink):" I would just like to go on record as saying you are pure EVIL, miss, and you are perfectly well placed in Slytherin with these horribly difficult questions. "
Slytherin and evil do NOT go hand in hand.
He was in GRYFFINDOR and BETRAYED his 2 best friends and caused their deaths.
The only house that does NOT have Death Eaters in it are Hufflepuffs, well, there are some Puff DEs but barely any to count.
Also, A LOT of Slytherins stayed to help Harry fight the Dark Lord at the end of the 7th book. It was only the children of the DEs aka Draco's circle of friends that left
Merlin was a Slytherin and the NUMBER ONE protector of muggles and muggle borns while he was alive.