Oda has shown us that there are two main justices: that of Akainu and other bad marines, absolute justice, and that of Garp and the good marines, honest justice. So Akainu must be defeated by these marines of honest justice not by a pirate looking for some foolish revenge even more so when he has other much stronger enemies to defeat (world government). Therefore, it will be Koby, alone or with the help of Helmeppo and Grus, who will defeat Akainu and his absolute justice, and not Luffy and Sabo who will fight against Im and the gorosei.