@The:
He hates people with a passion, but after Burning Ohara he was saddened seeing the giant tree burn as well as the others. He devoted his life to growing trees in honor of it. He feels no remorse for the ship of innocent people blown up, but the trees deserved better. As Fleet Admiral, he has a strict policy, kill people and save the trees. If you harm a tree, you get a lava fist to the chest. This is the real reason he killed Ace. Ace destroyed many trees with his fire power, which Akainu got angry at.
Well those people made boats and books out of innocent trees, he had no choice really. With all the tribal and flower tattoos you can tell he was just a peace loving hippie before the harsh expectations of an Admiral was thrust upon him. He just really cares about the environment thats all.