What's being attempted is clearer on a second reading, though I'm still lost re. the many pages of dog killing.
So the things Billy says are startlingly similar to those Eduardo does, something Eduardo forces him to recognize in their two meetings. Billy's attached to John, and to good, brave people in general, but doesn't have a view of the world justifying this - for him the world cannot be understood, and the only thing left to do is cling to personal comfort and safety, which will prove temporary even when they can be found. To do otherwise is merely folly, never admirable. He can't live by this philosophy, but it's there; similarly Eduardo can't either, as he simultaneously thinks the world is unfathomable and that the takers will prevail. In an echo of the Judge he calls his attempt to kill John an act of "naming," and clearly prides himself on thoroughly understanding how confused non-takers think. His own realization that there is no justice has made him embrace its opposite. That you can't know anything, strictly speaking, would be a self-erasing proposition; accepting it, then, you make some pragmatic division between levels of knowledge, denying only one - Billy lets himself help his friend, change tires for strangers etc., follows an ethical code basically, but refuses to admit it in speech. He claims to never have known what he wanted, in fact, but we know he once wanted very much to save an animal, then another, then a person. He claims to believe in what's in front of him, but he can't operate without reference to justice. But he can't admit or logically justify this, and that's a sickness.
So the point of the book is not that John is a hero but who he's a hero to: he saves Billy. This is symbolized by the puppy. Remember Billy's momentary refusal of compassion for a dog (itself referring back to his increasing negligence of the dog who'd loyally followed him and his brother)? John can't reach the last puppy - grabs a dead one instead - but Billy's arm is longer. Billy grabs it for John, but ends up taking care of it. The fate of this final dog is one of the many things elided in the sudden passage of fifty years, but I think we're to understand it's key to there having been fifty years, since it was the only thing he had left to care about. John gave Billy an example of an approach to life that made life worth continuing - proved to be the second brother the shaman had prophesied Billy would meet. He could neither accept nor refuse John's view of life, but even that kept him going, prevented terminal moral sickness like Eduardo's, left him ready for some final reconciliation.
The last Mexican he meets (and gives his only food to) provides that reconciliation by making him understand John's state of mind when sacrificing himself for what he took to be the good. Truly accepting the unknowability of the world means grasping what it is about it that you do know - that others are more or less in your shoes. Your plans, including your whole future life, are mist and rumor next to this small but present fact. That cowardice prevents most, maybe pretty much everyone, from acting quite as John and the Mexican's dream-hero do doesn't erase our need to contemplate such people and use them as our moral standard, a la "WWJD" - McCarthy is basically using Stevens' "Notes" position against Shelley's (at least momentary) assumption that the only people capable of mastering life were the principled world-leavers, like the legends of Jesus and Socrates (as compared to historical counterparts where who even knows what happened). To know what we should do is to know enough to go on, to center our values, even if we're prevented by our fear and other limitations from following through in a principled way.
The book's ill-structured, but the various cowboy vignettes mostly feed into this message: no one's exactly religious, but most have a sort of worship for a lost loved one who would always do the brave, right thing, enabling them to be a little braver and righter themselves. McCarthy tries hard to underline that many of these heroes are female, though of course he's much better at the gaucho stuff so the women show up as cameos or in anecdotes. He's also uncomfortable with the notion of females sacrificing themselves - seems like there's a separate salvation standard for women, Marian rather than Christy, where direct exposure to violence isn't required to teach them what's right - e.g. the cook whose name translates to "Succor." (One thinks of his bewildered dismissals of Proust and James - maybe stand-ins for female authors, who he absolutely never alludes to or mentions.)
No Country continues on from this pretty directly: Bell speaks of his wife in pretty much the same terms Mac does, with I think one phrase exactly repeated, and the breakdown of law embodied by Chigurh causes one of the two kids who let him go to get to where Billy and Bell are. The other one not so much - McCarthy does admit that violence and lawlessness create dickheads and psychopaths as reliably as it makes genuinely good people (which explains his two kinds of Mexican), but holds that high civilization creates neither - Cole's mother is tellingly an actor. Even that judge guy from AtPH is a Bell-type figure, after all, vividly remembering when the Southwest was effectively as lawless as mid-century Sonora: McCarthy might agree with Cather that civilization only exists in the initial gulf-shoot of civilizing, of ordering some new world. In part McCarthy's settings are extreme because he's looking for test cases, mocking up a kind of lab, but there also seems to be a thick vein of only half polite, almost Flannery-worthy contempt for the Northern, Eastern, modern, prosperous, safe in the post Blood Meridian books. After a thousand pages of Border Trilogy that gets a little wearing, whereas with O'Connor you're mostly amused to be dismissed as Satanic, maybe even a little flattered, like Europeans surely are when reading James.