percy saved the ophiotaurus' life in-text not solely on humanitarian grounds but through logical reasoning. in theory, just to be safe, the gods should have killed it. whoever kills it and sacrifices its innards will gain enough power to overthrow the gods, so they should have killed the ophiotaurus to protect themselves, and while they were at it they should have killed percy too, and they should have killed nico, and they should have killed thalia again to prevent the prophecy from being fulfilled, just like kronos was right to eat his children to avoid having his power usurped, just like laius was right to try and murder oedipus to avoid being killed, just like king acrisius was right to get rid of danaë to avoid being killed by his grandson perseus - except that doing so were acts of cruelty that never worked in their favor, which percy rightfully and successfully used in his argument to save the ophiotaurus. bessie is percy in this scenario, and he's also oedipus, perseus, the eldest olympians themselves, and every other greek figure who was targeted with cruelty and violence out of fear of what they might do, without regard for the intrinsic value of their lives
hermes understood luke's fate well before it even happened, and he knew very well that he could never have prevented it, which is why he largely gave up trying to do so after efforts to keep may from becoming the oracle didn't work. this is why it's so necessary to understand percy's role in the narrative as a breaker of cycles, and why it's necessary to comprehend that role in context of the greater grecian mythos from which these books are inspired. in greek mythology it's so commonly the nature of a tragedy that some terrible thing is always doomed to happen and efforts to prevent it or manipulate the outcome wind up being the direct cause of that tragedy, which is a crucial aspect of the "greek tragedy" concept in and of itself. percy understands this, and so he acts accordingly
the same book where percy fights to save the ophiotaurus is the book where he decides to finally claim the prophecy for himself. he is different from thalia, whose rebirth was, in itself, an inorganic scheme explicitly intended by the titan army to manipulate the prophecy's outcome in their favor. where thalia's dangerous bloodlust for the ophiotaurus validates the gods' fears, percy's concern for its innocence and right to live contradicts those fears. where thalia (rightfully) avoids becoming the prophecy child, percy stops resisting it because he understands that it has to be him. percy's empathy for bessie, for nico, for his fellow demigods is the reason why he can finally alter this mythos. this is why he is the protagonist and the most essential figure in the narrative, this is why, in handing luke the knife, he becomes the ultimate agent of change but not the tragic greek hero that luke was, whose alignment with kronos, one of the originators of this exact cycle, was always flawed and always doomed to fail
percy ends the series with a final acknowledgement that making an attempt to exert control over fate is an inherently flawed methodology. "you've got to stop trying to get rid of powerful demigods" and "children [...] will be welcome and treated with respect" is an indictment against the cruelty of this long long history of fear-based, self-serving, preventative violence, and emphasizes that the prophecies surrounding these mythological figures of the ancient past (as well as the demigods of the present) were only told because the cruelty exerted to subvert these prophecies is the very element that manifested their existence
