The pass word to get into the Slytherin common room was 'pure blood' remember when Draco led Ron and Harry in (as Goyle and Crabbe)? The Slytherin's are obsessed with the idea of 'pure bloodedness' like the Nazi's were with with Aryian race (Jo makes this comparisson in an interview).

I know I've said this disjointed thought before but Voldemort's half/tainted blood is strange:

He's not ashamed of it; he chose Harry because of his half blood. Therefore, Voldemort thinks that there's power in the half blood; he knows that it's harder for half bloods in the magical world and, maybe, assumes Harry will be ... what: hardened (???) by this..???.... therefore, more of a threat/ more powerful than Neville because Harry is more like him, Voldemort...???

Voldemort, for all the reasons above, is also proud of his half bloodedness...??? An inferiority AND a superiority complex at the same time!!!> A superiority complex about the inferiority of his blood...?????

We are meant to find out more about Tom Riddles birth at some point, maybe that might explain a little more about the psychology of Voldy.

I reckon some Sltherins will come over to Dumbledore's/ our side, now the wars on. Remember that even the Deatheater Regalus (I think???) / Sirius's bro., tried to pull out when he saw what was expected of him. I think a lot of Slytherins like to talk 'pure blood' and act all elitest but when the war and killings start a happening, they'll choose the side of reason. What do you reckon????