Dragon being a former WG man is making more sense. He's Garp but with less tolerance to the crimes of the WG. Him seeing first hand what they did to Ohara and its people made him switch on them.
Something about "changing them from the inside" to "destroy them at all costs".
Or he used to bang Robins Mom, hence him looking out for her during the timeskip. Luffy really is Robin's lil bro