No, he never killed anyone. The reason why he never kills anyone is that he is leaving them another chance to achieve their dreams. This was also stated by Oda in SBS vol. 4:
D: How come Luffy never kills his enemies? Throughout "One Piece" manga so far, he didn't kill Mohji and Helmeppo, but Axe-arm Morgan was killed by Zoro, why is that?
O: Hmm! That's a very good question. First, I have to announce that Morgan is still alive. He's currently in jail, where he was placed by his former subordinates. Why doesn't Luffy kill his enemies? Because in that era, everyone uses their lives to fight for their dreams. For an enemy, when their dream has been shattered, it is the same as losing a fight, and as painful as death. I believe, for a pirate not to kill an enemy, it's giving them a 2nd chance to fight for their dreams.
