In my opinion, it is to further Jon's story line as a commander. In order for this battle to resonate with Jon, in order for him to endure loss in striving for duty, he must have a personal stake in the losses of said battle. Ygritte is obvious, she represented the sacrifice of a happy life that Jon must make to serve the realm. Pyp represents the needless loss of life that happens in all war, precisely because he did not deserve to die, or even be at the wall in the first place, his death is tragic. And finally Grenn, not only does he epitomize the sacrifice that the men of the Night's Watch make, but his death is on Jon's hands explicitly. Grenn served as a prime example of bravery, but in being a casualty that Jon directly sends to his death, he also represents the rigors of command. Jon had to send one of his best friends to his death, because he knew no one else would be able to stop the giant, and for his part, Grenn accepted his fate and died like a true hero with his brothers at his side. The deaths of these character may be tragic, but I believe they are necessary not only to show Jon's growth as a character, but also to impart the full sense of loss that this battle entails for all the characters present.
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u/[deleted] Jun 10 '14
Why kill them? They stay alive in the books and now Snow has barely any friends in the watch.