Every once in a while I think again about the end of Thriller Bark and feel completely insane and ill about Zoro's sacrifice, FOR LUFFY, specifically (you know, the character Kuma's threat was directed at). It wasn't even that long into traveling together, a few months maybe, yet Zoro was ready to give up everything in that moment - in the chapter with Kuma appearing being titled The End of the Dream ! - to protect his crew and Luffy, so he could continue in his journey.
Since Luffy and Zoro met, they always understand how words and promises were imporant to them - with Luffy punching Helmeppo in ch. 3 for lying to Zoro. Zoro learnt how serious Luffy was about his dream, and soon he realized he backed up his words with actions as well - untiying Zoro and giving him his swords back - his biggest treasures. It meant that Zoro could be honest and honor-bound in the same way to Luffy, to gain this mutual respect and trust between them since day 1. To wield his swords to protect both Luffy and later their whole crew, and to step in a way between Luffy and danger.
He was being actually more upset that Sanji got up from the previous Kuma's attack and interrupted their fight - Zoro was trying to keep the whole crew safe by this exchange for Luffy's head - if Sanji was offering his life for Luffy half dead and without much strength left to fight for himself (he started the offering of his own life already believing he woudn't survive, with a "you should find a new cook"), then this very specific sacrifice would be meaningless to the crew (- if this arc was taking place post WCI, then it would turn out very differently, with the strength of Sanji believing in Luffy, but it wasn't his moment during this scene) - it would hurt them more than help them, because as much as Zoro was prepared to die as well, he was prepared to keep fighting until the last breath.
Zoro was thinking he might die - Kuma's words were pretty certain he WOULD die - but he still had the willingness and strength to take on the deal for Luffy, for his captain and his crew. ("if i die here, it just means I wasn't worth much to begin with" this line he says times and times again during the overall story, like in Rogue Town throwing Kitetsu and waiting if it would cut off his arm, up until standing against King in Wano "it's my power that was lacking", and all the other times he was questioning his worth - it's something he tempts the fates he doesn't believe in, to actually harm him, to take his strength away if he doesn't deserve to survive. and it's him saying he knows and accepts his own weaknesses - of not being strong enough (in comparison to Sanji in this example), and always fights through them.)
He threw away his swords, including Wado Ichimonji - literally throwing aside his and Kuina's dream, to compell Kuma into a duel (with the anime playing 'The Very Very Very Strongest' when Zoro bowed down and pleaded Kuma, offering him his head instead of Luffy's) so Kuma wouldn't go after the crew and specifically Luffy later - no matter the outcome if Zoro would surive or not.
And then, he was actually strong enough to survive taking his captain's fatigue, agony and pain! Possibly being the only one who could survive taking Luffy's pain.
Zoro could have back out when Kuma offered him the 'taste' of the pain, with the realization of the scale of the hurt with the very possibility of dying from it. But that wouldn't be Zoro now, would it? He accepted and took all of Luffy's pain so his captain wouldn't have to suffer or die, and when they found him afterwards, he still kept standing, tense with the fatigue but alive! (again, with anime adding the music of 'Luffy's Fierce Attack' to underline the importance between these two).
He was training for this since the beginning - to become stronger to shoulder the pain of his crew if necessary. (And not only that - he was preparing for that so another Kuina incident didn't have to happen). He was the first one to fight one of the Warlords before anything really began: his fight with Mihawk at Baratie really set the tone and his own goals to overcome - a glimpse to see on how much different levels the Warlords actually were in comparison to Zoro, Luffy and the others, and if they were supposed to beat them so Luffy could become the Pirate King, that always meant to be ready and to get even stronger than them.
(small spoiler for egghead, ch. 1102: seeing Kuma (a Warlord at that time) remembering this Thriller Bark event later, during Egghead arc, and thinking that even he might have passed out from the pain, makes it all the more meaningful that it was Zoro who took the pain and withstood it - establishing how high was the strength of his willpower, already before timeskip.)
There could be so many other nuances and details from these last few chapters of this arc, and even what this deal meant for the following arcs! Zoro was still in pain on Sabaody, and because of that the crew wasn't as strong as it could have been (not to say they would have a chance anyway, knowing what all was in the motion).
The next is the tragedy and beauty of LUFFY never finding out about this. Half of the crew knew: Sanji, Brook and Robin knew the details, but would never tell Luffy - and that shows their loyalty to both Luffy and Zoro (and Zoro's decision). Luffy woke up and first thing he did was to jump up and down, excited not to be weighted down by his injuries, and only seeing his swordsman being down with injuries so severe he was out more days afterwards, knowing that something else attacked them (him = Zoro), after he was passed out from the fight against Moria, brought down his mood (even if it's not much noticable, but the change into subtle worry is there in the few next chapters).
"I can't explain it either!" - meaning he was thinking about it too, possibly how weird it was for him to move normally after such long fight. We don't really ever hear/see Luffy thinking about something, except when it's mentioned how he came up with a solution or idea, telling us there's more to Luffy than just being straightforward in his goals and speech. With Luffy being sometimes very emotionally intelligent when he wants to be, he could have figured it out from all these other people in the room asking similar questions and deducing. Even Usopp was putting two and two togehter. We might never find out if Luffy actuallly knows or not. Luffy probably wouldn't ask Zoro directly, especially if Zoro wouldn't tell first and didn't want to talk about it
- because for Zoro, nothing happened! Nothing, that would compromise his and Luffy's first promise. For Zoro to become the Strongest he couldn't back down from the duel with Kuma (just like before with his duel with Mihawk at Baratie. When he's faced with something he swore to overcome, he can't back down or evade. Even back then Luffy understood that as he held back Johnny and Yosaku, but Sanji was perplexed how far Zoro (and Luffy) would go to reach their dreams). When Sanji was asking him in front of Kuma "What about your dream?" Zoro was still thinking about his dream- it was just that the context has changed, it changed into a journey. His dream is the most important thing, but it wouldn't mean much, if, when on his way to accomplish that, he would betray his other words and promises.