Why is Snape allowed to continue spying?

[firebird]

No: HPFGUIDX 131491

Good question.

LV must know that Snape is working for DD and the Order. When LV was 
sharing Quirrell's body, Snape made it very clear to Quirrell that he 
was dedicated to thwarting Quirrell's attempt to obtain the PS/SS for 
LV. At that point, presumably, LV was too weak to do away with Snape. 
Either that or he didn't want to arouse DD's suspicion by murdering a 
teacher right under his crooked nose. 

I suppose we have to assume that the other DE's who have had access 
to Snape at Hogwarts (Lucius Malfoy, Barty Crouch Jr., Karkarof, 
McNair) either similarly did not want to draw attention to their 
allegiance to LV or were unable to kill Snape because he is a more 
powerful wizard.

All that said, I'm not sure LV knows Snape is a spy. I think Snape 
does his spying long-distance, by means of legilimency. If he's 
skilled enough, he can look inside the DE's minds (perhaps even LV's 
mind) without their realizing it. I figure that in the days of LV's 
ascendancy, Snape hung out with the DE's and learned to get into 
their heads. Now he sneaks in undetected while remaining within the 
protective walls of Hogwarts. Actually, I can't recall his ever 
leaving Hogwarts, except to go to No. 12 Grimauld Place, which is 
equally heavily protected.

In short, Snape is smart, careful, skilled and powerful -- as well as 
thoroughly unpleasant.

firebird