Actually, I think the Slytherin students reasoned rationally yet happened not to get the right answer.
Slytherin thinks that Snape can get away with being horrible because he’s blackmailing Dumbledore, that Harry found out how Snape is blackmailing D, and that D now has to try to please both of them.
In actuality, Snape is horrible at Dumbledore’s direction, in order that everyone think Snape is blackmailing him, when actually Snape is really on Dumbledore’s side (chapter 77). (Or at least D thinks so, based on love-for-Lily.) But Dumbledore really does have to keep Harry happy to some extent, so he directs Snape to be horrible to only half the students. Then D can maintain the fiction that Snape is blackmailing him, and can pretend that Harry is now blackmailing him too by finding out the same secret Snape has.
D plays along with Harry’s guess that he wants an evil potions master, so that he doesn’t have to tell Harry that Snape is secretly on his side.
Actually, I think the Slytherin students reasoned rationally yet happened not to get the right answer.
Slytherin thinks that Snape can get away with being horrible because he’s blackmailing Dumbledore, that Harry found out how Snape is blackmailing D, and that D now has to try to please both of them.
In actuality, Snape is horrible at Dumbledore’s direction, in order that everyone think Snape is blackmailing him, when actually Snape is really on Dumbledore’s side (chapter 77). (Or at least D thinks so, based on love-for-Lily.) But Dumbledore really does have to keep Harry happy to some extent, so he directs Snape to be horrible to only half the students. Then D can maintain the fiction that Snape is blackmailing him, and can pretend that Harry is now blackmailing him too by finding out the same secret Snape has.
D plays along with Harry’s guess that he wants an evil potions master, so that he doesn’t have to tell Harry that Snape is secretly on his side.