I'm baffled by how many people are interpreting Law's situation as a tensionless development.
As many have pointed out, Law will more than likely survive, but Law being on the verge of death matters more for how it affects Luffy and Doflamingo at the onset of their climactic fight than for what it means for Law. I don't think the story's intention is to fake readers into fearing for Law's life. It's more about adding weight to what the ultimate resolution of the current story (which started in chapter 655) will be.
! The story's current narrative arc, which began with the Straw Hats' arrival to the New World, has largely focused on distinguishing the humane from the inhumane; the story seems to be trying to show us where the line is drawn between these two qualities.
! The story has brought this exploration to light through key characters like Kyros, Caesar, and most importantly Law. Luffy and Doflamingo, of course, stand almost exclusively at opposite sides of these qualities.
! Notably, Caesar and Doflamingo gave a similar speech at the height of their control in their islands. In 689, Caesar wonderfully sets up the story's task; as he describes the unique power he holds through Punk Hazard (P.H.); he goes off about how those that arrive at P.H. can run, can scream, but nobody will come to save them. In 746, as Doflamingo lays out his game, he says the same things to those in Dressrosa–they can scream, they can cry, but nobody is saving them. Ironically, Bird-Cage and Shi-no-Kuni, the manifestations of Doflamingo's and Caesar's powers and wills, trap them as much as they trap their victims. In this way, we see how the inhumane violence that these two spread over their sphere of influence in the world is as damaging to them as it is to others. And yet, it is clear that these two hold a very significant grip on the New World.
! As such, we ask ourselves: why should we choose to act humane when those who act inhumanely can hold all the power?
! Law is very personification of this question. The fate of Law -- his survival or his death -- determines our answer.
! Through Law, we see the struggle that is being compassionate in a cruel world. His backstory speaks for itself. In 762, as Law prepares to stab Corazon, he asks himself why miserable people like Corazon (who for Law at this point symbolizes the cruelty of the world) should live while caring people suffer horrible deaths, like those of his family and friends in Flevance. Law himself is on the fence between humanity and cruelty. In 752, Law says he wants to not be like Doflamingo -- Law wants to escape his association with inhumanity.
! This desire has many aspects. Law is called a monster as a child. By joining with Doflamingo in his childhood, he seeks the same kind of wanton destruction that Doflamingo seeks. All the same, the influence of Corazon deviates him from this path of cruelty and introduces Law to the opposite of his wanton destruction: random kindness and compassion. Corazon saves Law for no other reason than the human feeling of empathy, as he states in 764. Through this relationship built on empathy and kindness, Law manages to prolong his lifespan. He survives his illness and gets to grow up.
! Interestingly, in the present timeline, we are told Law's reputation is one of cruelty. Apoo alludes to this reputation at Sabaody, and again we hear it from the G5 marines upon Law's reintroduction in P.H. in 659. Yet we also see Law act on kindness, both when he takes Jean Bart into his crew (helping him through a lifetime of pursuit from Celestial Dragons) and when he saves Luffy at Marineford. Thus, at present, Law is still very much on the fence between his humane and inhumane qualities.
! The ultimate answer to Law as a character, then, is what kind of person he will ultimately be; this is determined by the resolution to his character arc--the fulfillment of his quest for revenge. When Corazon dies, Corazon cries out for Law's freedom; he wishes more than anything for Law to leave Doflamingo and piracy behind and to simply live a long happy life, free of the inhumanity he suffered as a child. And yet, as Doflamingo poignantly says here in 780, Law has not done that. Law has dedicated his life to the sole pursuit of Doflamingo and piracy. In Law's mind, as he says in 768, he can only be free until Doflamingo dies for his crimes. Yet, like Doflamingo alludes to in this chapter, Law has essentially thrown aside the whole reason why Corazon died in the first place.
! Pulling it all back in to Law's current critical state: this is by far the most tense point of this narrative! Not for whether Law lives or dies, but for what it means if he lives or dies.
! The answer to Law's character will come through the outcome of his inner conflict ultimate fight. Doflamingo represents the inhumanity of Law's life; Luffy is his humanity. Law saved Luffy through humane compassion, got Luffy to call him a friend, and places absolute faith in Luffy –- the same faith he places in his being able to renounce cruelty and finally be free of his inhumanity. In this chapter, the roles that Luffy and Doflamingo play in Law's narrative were cemented. Law wholeheartedly declares his belief in Luffy (he makes miracles!) and gives the definitive middle finger to Doflamingo. Now, in the most vulnerable spot he's ever been in, Law will lay witness to the physical manifestation of his inner conflict.
! For Luffy, who is hell-bent on never letting someone in his life die again, Law's current state is the fuel he needs to defeat Doflamingo. Law has explained to Luffy why he went after Doflamingo (Kaido plan was a lie) and told him (in 749) of Doflamingo's crime: he killed his own brother. Luffy, having just come back from losing his own brother, has every reason to dislike Doflamingo. Now, with Law (and Bellamy and all of Dressrosa) in harm's way, Luffy is dedicated to finishing Doflamingo, and everything he stands for, off.
! For Doflamingo, Law's allegiance was always about validating his own life choice to stand against the world at large. As he says, he sees himself in Law. But when Corazon feeds Law the Ope-Ope, Doflamingo is angered at how now he must ask Law to die. Even further, he is angered at how Law makes clear that he wants nothing to do with Doflamingo's way of life and his way of being. This is what 752 is entirely devoted to. Doflamingo, though fully capable of ending Law's life at any time, has pulled his punches and reneged on killing Law at every turn, all in the hope that Law will simply agree with him again. Now, Law having finally given Doflamingo the middle finger and professed his devotion to Corazon (his coat, his crew's name, everything), Doflamingo needs to have Law die. His wish for immortality is a way of him wishing to immortalize his ideals: in 761, he tells Luffy humans are savage creatures by nature; in 725 in Momo's flashback, Doflamingo says the same when punishing the gladiators who pulled punches during a colosseum fight. (And of course, the source of Doflamingo's own stance in the story's humane vs. inhumane stems from his father's declaration of humanity, but that's another essay)...
Thus, the battle to repel Doflamingo, and all of his ideals, is about to start! And Law's fate being in question puts the story's thematic resolution in question. –--> TENSION!
