Caribou's father ruled over Hachinosu. Caribou probably thought his whole life he would one day inherit it. Then comes Teach and crushes that dream. How will Caribou pay him back for that? He should know that he can't beat him, no matter what sceme he concocts. Even stabbing Blackbard in his sleep wouldn't work, the guy doesn't sleep. Only thing he could do is to only feed false information to Teach, but that would risk his own life. Better would be to give him true information and mix in falsehoods: Poseidon is at Fishman Island and Pluton is on Elbaf heheheh.