It's kinda funny how in this incarnation of One Piece, the bad guys outright die. Even in the original Japanese versions, Captain Kuro and Janko are just knocked out cold (very solidly, mind).

Of course, there are reasons. In the original, they wanted to keep them around just in case they had more stories they wanted to tell with them later down the line (Not to mention the original characters weren't that brutal). While D&D players, while not completely heartless, simply do not give a damn about the sanctity of life while playing a campaign (Unless they need to). If they were bad enough, they should die, that's that.

Just thought it was interesting how this is the only incarnation of this rather violent series where the body count actually piles up.

Actually it's because Luffy thinks having to live with being defeated and your dreams shattered is a fate worse than death. So yeah, he's NOT being merciful when he doesn't kill an opponent.