How many people died because Harry...
didn't use the Killing Curse when he had the chance?
In the Department of Mysteries he uses weak stun curses on several Death Eaters, including Dolohov, one of the original Death Eaters, who he later spared again. Then he used the Crucio curse instead of the Killing Curse on Bellatrix.
After Dumbledore was killed, he uses weak stun curses instead of the Killing Curse on the Death Eater in charge who wasn't named, Fenrir Greyback (mass murderer) apparently right before he attacks Bill when he recovers, Amycus Carrow and the big blonde Death Eater, who again he later spared.
So do you think it was dozens or hundreds who were killed because Harry was too much of a pussy to use it?