Ok so you think he using Luffy for something. Ok got it
He stole the Nika fruit, clearly knew what it was when he fed it to him since he recognizes his poster, and ONLY just decides to go after One Piece after seeing it.
Once again, this arc is showing that Luffy's heartbeat powers ancient tech, its not that hard to imagine that Nika is needed to power something (i.e. One Piece if its a weapon or tool or powerful object).