During his fights, Luffy frequently sends the enemy flying, knocking them out or beating them until they have reached the point that they are almost near death. Neverthless Luffy never kills any of his enemies (no matter how cold-hearted they are).
Oda explains that it is not a question of morality so much as a matter of punishing the villains for their crimes – he feels that killing the villains lets them off too easily, whereas he considers letting them live to see their dreams ruined a far more fitting punishment:
Why didn’t Luffy kill anyone? In this era, people put their lives on their beliefs and convictions, and fight. When he goes into battle, Luffy is destroying others’ beliefs. And when these enemies’ beliefs are shattered and defeated, they feel a pain equal to death. I think that for these pirates, killing or not killing is secondary to winning or losing. (Oda)
However, it is evidently clear that Luffy is not against others completing the final act themselves, as seen when Luffy allowed Law to make his attempt to kill Doflamingo, while making sure Trebol did not interfere. This may have been more to do with stopping Doflamingo’s birdcage (which would only deactivate if Doflamingo willed it or was dead or unconscious) from killing civilians than actually harboring any desire to see Doflamingo killed. Ironically though, some of the people Luffy has defeated have become good after their defeat.