My thoughts:
Vash and Knives are brothers, and they fall upon a planet where humanity is just beginning—is in fact, falling from grace, er… the sky. The fall of Project Seeds from outer space onto the surface of Gunsmoke is humanity’s fall from grace; they lose most of their technology and are forced out of their comfortable living onto a harsh planet where sorrows and travails abound. Knives instigated this fall; he played the serpent. Though the serpent of Genesis is never actually proven to be Satan in the Bible, this is a common theological assumption that works well with my point. Satan was once the most beautiful and proud of the angels before he fell from on high demanding to be considered better than anyone else, even God. Knives also seems to consider himself superior to all the other lifeforms he has encountered, and betrays Rem (who is in a double role of sorts, identifiable with both Eve and God). They call the gun-transformed, ‘Angel Arms’, and until near the end it is not that apparent that Knives and Vash aren’t angels (or at least it wasn’t to me).
Once on Gunsmoke, Vash continues to travel with his obviously deranged and deluded brother, though he does have some very unVash-like thoughts. In one scene, we are shown Vash standing over a sleeping Knives holding a large rock and obviously wanting to bash his brother out of existence. He wrestles with his conscience. Vash here is playing the wrong role; he has somehow slipped into Cain (Genesis 4:8, “And Cain rose up against Abel his brother, and slew him”). Vash’s murderous desire stems from wanting to avenge Rem and the rest of the crew of Project Seeds, whereas Cain’s reason was pure jealously. One wonders if Knives isn’t closer to this role, since the subtext of his murder of Rem seems to be jealousy—both that Rem loved Vash more that she loves him, and that Vash loved Rem more that he loved Knives. (Remember: Cain killed Abel because God liked Abel’s sacrifice more than Cain’s. In this case, Knives didn’t kill his brother, he killed the God-symbol, Rem.)
The final battle between Knives and Vash takes place in an oasis, a very rare place on this desert planet. It is interesting to note that when Wolfwood talks about wishing to be reborn in Eden, the place he pictures is a similar setting. This suggests to me that Vash and Knives has to ‘go back to Eden’ to settle their differences, and that there really is no hope for a return to innocence on Gunsmoke. Humanity has already fallen, and they cannot reenter the garden. All they can do is be redeemed, which leads me back to my essay on Vash as a Christ-figure. He will continue to wander the world and assisting other people, hoping for the best.