The nidai kitetsu is the sword that luffy took without asking for permission. It was said that who ever wield this sword would have a horrible death.
Or would it? This is what i think!
this sword the nidai kitetsu was actually the sword of zunisha back when he was a human the sword was cursed along with him because probably the sword was a part of him because he is a samurai.
just like the other swords like enma, and haberokimi i think this sword is alive, and know who its master or someone related to its master i think luffy is one of this master who had the right to open this sword.
so when luffy did open it something probably triggered. Like a power perhaps luffy can connect more to zunisha himself. He could probably order him around now. Or talk to sea kings. Or even that the nidai kitetsu is the key to open up the ancient weapon uranus.