@robbybedfart:
Luffy already had the Bellamy incident to showcase the fact that words means nothing, and he knows better, but actions mean everything.
Also. When Luffy was on an execution platform, he was cheerful about it.
In any event, Ace lost a lot of respect with all his whining and moping later and his "I don't want to live" and "You guys get out of here, I'm worthless!" The exact same scene applied to Robin worked so much better because she was intentionally sacrificing herself for them. And then the moment she saw the entire crew was there for her, she realized she wanted to live, and started fighting back and resisting as much as she could.
Ace… just didn't want to be rescued and was resigned to die until the second he was uncuffed.
Ace was just being snotty and yes... emo. Self destructive, world hating, no reason to live. Emo.
There was no one at risk when he was up on that platform but himself.
Yes, Luffy knows how to act when in non-trivial situations, and yes, Luffy was laughing up on his education platform, but there are subversions to both situations.
Luffy is a hard-headed character who learns by experience. Even if something is a terribly bad idea, he'll try it once first before succumbing to wisdom.
Case in point, Magellan. Luffy shouldn't have gone up against the warden (he'd been warned not to) and should've instead tried to find a way to escape from him, but he was having none of that. Because of this, he missed his chance to save Ace before he was taken away. On all subsequent encounters, he knew to run away. The same thing goes for other enemies. Smoker he tried to fight - realized he couldn't after getting taken down. After that, he advised the crew to just run away if they saw him. Aoikiji, same thing. Tried to fight - lost. After that, advised his crew to run away from the Admirals when Kizaru showed up.
Luffy shouldn't have fought Blackbeard, but did anyway because he lost his temper. You can be absolutely, positively sure that if Ace had just rescued him, and a bleeding, dying Shanks was in Whitebeard's place having his entire life as a pirate be spat on and trashed right before his eyes, Luffy wouldn't thought twice about trying to defend Shanks' honor. He wouldn't have been thinking straight - his mentor is dying before his eyes for his sake.
I'm not defending Ace, I'm just saying Luffy would have lost his cool just like any other situation where he lost his temper and came out fists swinging. -And Akainu would have killed him.