Imu is retarded, he could have teleported to Sabaody, Domi Reserved Luffy and command Luffy to exterminate his crew and than buster call the entire island or use the Mother Flame to blot out the island from under heaven without leaving a trace.
But no! He laid back and became stubborn and arrogant, thinking I have this, they will not prevail over me.
But his doom will come and he deserves it.