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No, not all. Although, the majority of the Slytherin students believed in pure-blood supremacy, there were some who didn't like Horace Slughorn , Andromeda Black and other Black family members.

After Voldemort's defeat, the Slytherin house wasn't all 'pure-blood for the win' , but they still had an affinity for the Dark Arts :) Also, severus snape was in slytherin, and although ha had hi faultls, he was all in all a good people.

There was not a single Slytherin student who remained in the castle to fight Voldemort and the Death Eaters. Voldemort said that all the Slytherin students joined him. Group pressure? (Draco Malfoy, Crabbe and Goyle had stayed behind to try to capture Harry for Voldemort.) Whereas a number of the seventeen and eighteen year old Hufflepuffs and Ravenclaws stayed behind to fight. And there were so many Gryffindors staying that McGonnagall had to force the underage to leave.