So, first let's think about what kind of guy Figarland Garling is based on his appearances in the manga?
First, we can see he is very quick to kill people and enjoys doing it for fun. In the god valley hunting games, he is quick to kill the king and does so before the games even start, this causes him to lose points
So based on this, we can say he loves killing, and is impulsive and quick to act. This is very different then most celestial dragons, who are generally cowardly and just want to be rich all day, they don't wield swords and kill people so casually. Garland also doesn't have a space suit, like the other celestial dragons, showing how he is not just an aristocrat but actually a Rob Lucci type character who enjoys killing...
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So this brings us to the question, why did Garland not kill anyone in 39 years of the OP universe. One may start with the Rocks pirates,
So, as far as we know, none of the relevant characters would count as someone Garling killed or even seriously injured. Rocks himself was taken down by Roger and Garp as per anyone.....
Whitebeard, Kaido, Big Mom and Shiki all escaped to become Yonko later on...... so he couldn't have killed any of them.... So while we can accept maybe he killed one of the unknown Rocks pirates, he did nothing to any of the important ones, who all escaped. Another group that escaped were the slaves at God Valley, who were also supposed to be hunted and killed.
However, the slaves escaped, so he can't be blamed for failing to kill them, right? I mean to some extent yes, but one of those slaves returned to attack the celestial Dragons island during the Egghead arc.....
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At this point in time, Kuma or the "Buccaneer brat" attacks. Now at this point in the story, not only did Kuma escape, he joined the revolutionary army, lead by Monkey D dragon. These are very relevant groups according to the celestial dragons. They call the Bucaneers people who have committed a great sin against the world.
Now, keep in mind, while they say against the world, it is very likely they mean against the CD's, or only they can remember, because as far as I know, no one really discriminates against Kuma for his race aside from them.
To signify how serious the anti Buccaneer discrimination from the celestials, Saturn even says that NOTHING else can be tolerated.
Now, with Kuma working with Dragon, we can remember that the Cd's literally grow up hearing about the will of D people are their natural enemies....
So anyway, in this serious moment why would Garland not be here and ready to kill Kuma? He is the natural enemy of the celestial dragons in more ways then one, and Garling enjoys killing so much that he loses points to do so, and yet....... he is nowhere to be found as Kuma attacks.....
and this isn't really a coincidence, he was not present during the Revolutionaries attacking the holy land. He only shows up afterwards to kill mjosgard, who was not even the most serious traitor during this event(Fujitora was, and he walked out unharmed). We also see that Garling did nothing to stop Fisher Tiger, after he went around the Holy land freeing thousands of slaves and causing fires.....
And again, the Holy Knights have teleportation and Fisher Tigers general location was being tracked by the Marines, so there's no reason why someone like Garling wouldn't be able to come and kill him, since we know he loves to kill people.
This brings us to the most likely and reasonable conclusion, Figarland Garling.... is a pussy and a bitch who doesn't have the balls of ChouChou the dog, who will die to defend his home.
again, one piece is a series where real men do not fear death and since this is a kids story, it's not exactly nuanced. Imo, this explains a lot about the story. For example, what do the Gorosei say to Shamrock, Garlings son who remained a CD?
"this is the reverie, given your standing.... you should have nothing to do with politics". One of the Gorosei does not even look at him.... Can you imagine the Gorosei talking to Charlos this way? Probably not, in fact it is extremely rude. This makes sense when you consider the fact that the Gorosei probably do not respect Garling for being a pussy that they say his son should have no political influence and that they would rather meet with King Cobra........