Why Is Snape Alive???

[Ask and I may tell]

No: HPFGUIDX 107039

 Hello everyone, New gal here. I am sure there are some passages 
that might help me with my question but I have yet to find it. Ok 
here we go......
 I know that Hogwarts is a well protected place. It has been said 
many of times that Harry cannot be harmed while at Hogwarts. And it 
has been said that anyone who has turned their backs on LV has been  
or will be punished. Why is Snape still alive? Why is Malfoy still 
buddy buddy with Snape? .... Ok so Hogwarts is protected, is everyone 
who is inside of Hogwarts protected? Does Snape ever leave the school 
or just stay to be  protected? Why are Snape an L. Malfoy still 
friends? This to me does not make sense. In OotP Malfoy seems to be 
one of LV most loyal followers. Why haven't they gone after Snape for 
turning his back on the Dark Lord? Could Snape be the one spying on 
DD for the Dark Lord. It's late and maybe I am not making sense but 
some of these questions I have searched and searched and haven't found 
any answers. 

Thanks for listening to me babble...Have a Great Night!!

xoxo, Peach