Why does Snape save Harry for the Dark Lord?

[carodave92]

No: HPFGUIDX 137142

I apologize - I lent out my books and can't provide quotes.  

In HBP, during his flight from Hogwarts, Snape tells the DEs to leave 
Harry for the Dark Lord.  But in GoF, Fake!Moody tells Harry in his 
office that when he (Fake!Moody) kills Harry, he will be rewarded  
beyond all others by the Dark Lord for completing the task that the 
Dark Lord so wanted to do (but failed).  Does LV suddenly want to be 
the one to AK Harry, even if it means that Harry lives a bit longer?  
This seems to back up the theory that Snape is actually on the side of 
good, if not, why wouldn't he AK Harry and be rewarded by LV?

Carodave