Ace was there to be executed.
What did he expected? The marine to say SIKE and free Ace?
He could have done something to save him ,he couldn't and reacted when it was too late.
Yeah that wasn't the problem, his problem is whether he should save Ace or not.
Because on one side, he loves Ace and doesn't want him to die.
But on the other side, if he saves Ace he's gonna be expelled from the Marines and now both are being hunted.
"His" child was murdered in front of his very eyes. The emotions took over him as his body was literally moving on its own. Garp is incredibly strong probably could've gone either way with the weakest Admiral but ultimately he suffers from the same old age as Old Ray did. Their era ended long ago.
Akainu isn't the weakest Admiral lol.
But seeing as Sickbeard could handle him I bet Garp could.