Leo had originally thought the first to surrender would be Marines like Aokiji, who had no strong factional ties, or Marines like Kizaru, who had been captured early and subjected early to the Revolutionary Army's ideological influence.
But he had never expected Akainu to surrender.
He could roughly grasp what was in Akainu's heart with a little thought, but before Akainu actually did it, no one believed Akainu could do such a thing.
Yet after he did it, everyone could instead understand him and feel what he was going through.
He was not actually that different from Zephyr. Zephyr had saved Akainu with his own life precisely because he understood Akainu's way of thinking.
In the original story, Akainu had ruthlessly killed Ace purely because of their opposing positions. If Ace had joined the Marines from the start, Akainu would at most have disliked him; he would never have held any grudge against Ace—he only dealt ruthlessly with deserters, and Ace could never have been one.
Bloodline Theory was the stupidest thing—but under the Celestial Dragons' rule, everyone had no choice but to acknowledge its supposed correctness. Ace's father was Pirate King Roger, and then Ace had joined Whitebeard, one of the Four Emperors, becoming a division commander. That happened to validate Bloodline Theory, making people feel that once he grew up, he would become another Pirate King Roger.
But after Ace joined the Revolutionary Army, that notion was overturned at its roots, making everyone understand that dragons did not necessarily give birth to dragons, nor phoenixes to phoenixes, nor did a rat's son necessarily know how to dig holes.
Then the Revolutionary Army's overthrow of the Celestial Dragons' rule dug out the very roots of that idea: if Bloodline Theory was real, then why were your descendants all useless