Looking back, I think "Fire" should be above "Magma" like "Ice" is above "Snow", so when Akainu killed Ace, it should be that his Haki was stronger and that's why he killed Ace, after all Ace had been starved and tortured for a long time by that time, and he was also kind of weak to begin with, if you gave Akainu Ace's fruit instead of his own, I'm sure he could have been even stronger and killed Kuzan