As we know, Luffy hasn’t killed anyone, yet.
Oda explained why all Luffy’s former enemies are still alive :
D: Why doesn’t Luffy ever kill his enemies? If you look back, he didn’t kill Mohji, he didn’t kill Helmeppo or Captain Morgan, or anyone. Well, Zoro was the one who beat Morgan, but still. Why?
O: Ahh, good question. First of all, Morgan is still alive. He’s being held in prison now by his former subordinates. So why didn’t Luffy kill him? 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.