No, he never killed anyone. The reason why he never kills anyone is because he is leaving them another chance to achieve their dreams. This was also stated by oda in a sbs

>  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 doesnt 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.

http://onepiece.wikia.com/wiki/SBS_Volume_4