Snape as Head of Slytherin

[lipglossusa at yahoo.com]

No: HPFGUIDX 30705

Also, canon seems to indicate that the Heads of each House were 
actually IN that particular house when they were students at 
Hogwarts.  Perhaps Snape was simply the only teacher who was also a 
Slytherin (I say this because none of the other current teachers at 
Hogwarts seem particularily Slytherinish.)

If part of Snape's job is to watch for "Dark" signs in any of his 
Slytherins, he doesn't seem to be doing a very good job of it since 
his favorite student, Malfoy, is a prime candidate to be a future 
DE.  I have yet to figure out why exactly Snape likes Malfoy-- or 
gives the impression that he likes him-- it may be out of respect to 
Lucius or just the fact that he loathes Harry.  It's another aspect 
of his personality that keeps us wondering whether whose side he's 
on. 

In any case, I'd be interested to know exactly how long Snape has 
been a teacher at Hogwarts, who had the position before him, and why 
that teacher left.  Could it be that the previous Slytherin Head was 
caught grooming kids to join Voldemort?  Why exactly was Snape 
hired?  Chronologically, Snape appears to have been cleared of the 
accusations of being a DE just after Harry's parents were killed and 
Sirius was sent to Azkaban.  I'm guessing that he was not a teacher 
at this time, because in PoA, Sirius appears to be unaware that Snape 
is now a professor:

"Snape?" said Black harshly... "What's Snape got to do with it?"
"He's here, Sirius," said Lupin heavily.  "He's teaching here at 
Hogwarts." (PoA 18)

We don't know what exactly Snape did to prove to Dumbledore that he 
had truly left Voldemort's circle.  Perhaps Dumbledore hired Snape to 
prove his trust in Snape after vouching for Snape's innocence-- and 
incidentally securing for himself an ally at Hogwarts who REALLY owes 
him a favor?