Basically this^^and IMO he fooled the people of Kingdom with his DF(by controling King) ,I don't think he would carelessly use it b4 the people of his kingdom
Dramatic purposes most likely. I don't think you should be thinking about it too much. I also assume that Doflamingo probably doesn't show too much of his DF in public on Dressrosa to avoid giving people ideas.
I agree I think he doesn't want people to realize how his power works so they don't put together what he did the night he forced the King & his Army to attack the people.
& maybe he wasn't trying to kill Law maybe he was just torturing him.