User blog comment:Kaido King of the Beasts/Why Does Doflamingo Carry a Gun? - And Other Stupid Questions/@comment-24638955-20141125183600

The reason he uses a gun on someone is because when he uses his gun is to make stand out he is executing someone as a World Noble/Celestial Dragon (I know he technically isn't but for Doflamingo point of view he is still) rather than as a pirate which then he would use his DF powers or other means.