Wicked132
Wicked132:Why risk change the story when he can benifit from keeping it mostly intact?
I think jumping fully into piracy before the Grand Line is a waste so better he doesn't kill them yet. Though with Johnny's character the only thing he would be interested in exchange for letting them leave would be Kumadori's Seimei Kikan's method of training. Besides that there would be absolutely no obvious reason for him sparing them even if he is made into a pirate before the Grand Line.