Oh no, he's not. He's not from our world. He just reminds me of Sirius.
I think it's a little different for the two houses, and I'm talking now on a general basis and has nothing to do with this circumstance.
Gryffindors believe that when they are right, it's some greater good right that gives them some kind of moral justification. When a Slytherin believes something is right, he knows that the fact that it's right for him means that it's wrong for someone else. We already know that it won't be universally accepted.