The reason Kin might hold a grudge against dragons is most likely tied to Wano being terrorized by a dragon, before Ryuma defeated it and killed it. This is found in Oda's oneshot, called Monsters, where Ryuma, who looks a lot like Zoro, kills a Dragon.After doing so he became the hero of the island which most likely is now Wano. It was confirmed by Oda in the SBS that the Ryuma in the oneshot is the same Ryuma in the Thriller bark arc, effectively linking both stories. Ryuma is Wano's hero, and given how the story went in Monsters, it's because he saved the island (Wa no Kuni) from a dragon.He's most likely going to get the same backstory, but hopefully some things will be changed, mainly his appearance.
I find that the reason people want to believe that Kaido is a Dragon DF user, is because he was on a sky island, thus they think the only way to explain it is that he can fly. However back in the Skypiea arc, Gan Fall revealed that there are two ways to reach the sky islands, and that is the knock up stream and the Summit of High West.We don't know what the latter is, but it at least confirms that there's another way to reach sky islands, other than the knock up stream.