1. Higuma's malice is directed solely at Shanks, he makes no threat or insult towards Makino 2. You're right Luffy is not a hero, he will not share his meat but there is a certain way through which he wants to become pirate king. If what you claim about Luffy becoming pirate king is true then when Rayleigh offered to tell him where the One Piece was back in Saobondy Archipelago he would have taken him up on his offer. If what you claim about Luffy becoming the pirate king is true then he would not have had such a dissatisfied look on his face when Ussop suggested that they could sneak and steal their way to acquiring the info on the Road Poneglyphs that lead to Raftel. 3. Drum Kingdom, Alabasta, Skypiea,
Thriller Bark, Saobondy Archipelago, Fishman Island and Dressrosa are all examples of 'random countries' in which Luffy intervened because 'some random dude' was mistreating and or enslaving people there. It was no side effect, it was his express intent. 4. Outside of any personal grudge or relationship he may with them, Luffy is intent on picking a fight with the Yonkou because he wants freedom. Luffy is well aware that the Yonkou rule over the New World, as such he knows that he cannot be pirate king and free if there are other pirate powers ruling over the seas he wants to sail as "the freest man in the world". Therefore it follows that he must defeat all of the Yonkou, not because that is what is required to be pirate king but because, to reiterate my original point once more, that is the way in which Luffy wants to become pirate king.