I was wondering this question when a friend gave me the answer Oda himself actually answers this in a special Q&A section at the end of the 4th volume
for some reason the picture isn't showing up so I'll just put it up here
Q: How come Luffy never kills his enemies?
Oda: HMMM! thats a very good question, First I have to announce that morgan is still alive, he's cureently in jail placed there by his former subordinates.
Why does luffy never kill his enemies? Because in that era, everyone uses there lives to fight for there dreams when there dream has been shattered its the same as losing a fight and as painful as death. I believe for a pirate not to kill an enemy is giving them a 2nd chance to acomplish there dream.
(I paraphrased the question since it asked alot of questions that we already knew such as about morgan and buggy)