I don't believe what Coby did makes him a total loser but it was extremely naive and hinted that he had never taken part in an battle before.
I think most people on this forum would say Monkey D. Garp is a good man and has morals but you don't see him jumping in front of the most ardent and possibly strongest Admiral, telling him to stop the killing.
Coby will eventually learn that there are other ways to go about things then doing what he did.
What he did was brave but brave isn't always synonymous with smart.