In the end it's hard to tell whether it's Snowden's or Nately's death that finally inspires Yossarian to defy Cathcart, but it's certainly Snowden's death that haunts the novel:
There were too many dangers for Yossarian to keep track of. There were bartenders, bricklayers and bus conductors all over the world who wanted him dead, landlords and tenants, traitors and patriots, lynchers, leeches and lackeys, and they were all out to bump him off. That was the secret Snowden had spilled to him on the mission to Avignon-they were out to get him; and Snowden had spilled it all over the back of the plane.
There's no doubt that death haunts Yossarian, but why wouldn't it when dying is the only way out of war. Send people out on enough missions, and they're bound to die. Fortunately, though, Snowden reveals more to Yossarian than just the inevitability of death:
Yossarian was cold, too, and shivering uncontrollably. He felt goose pimples clacking all over him as he gazed down despondently at the grim secret Snowden had spilled all over the messy floor. It was easy to read the message in his entrails. Man was matter, that was Snowden's secret. Drop him out a window and he'll fall. Set fire to him and he'll burn. Bury him and he'll rot like other kinds of garbage. The spirit gone, man is garbage. That was Snowden's secret. Ripeness was all.
It is the human spirit, not mere existence, which gives life meaning. Of course, despite this earlier realization, Yossarian still seems to accept Cathcart's proposal:
"You see Yossarian, we're going to put you on easy street. We're going to promote you to major even give you another medal. Captain Flume is already working on glowing press releases describing your valor over Ferrara, your deep and abiding loyalty to your outfit and your consummate dedication to duty. Those phrases are all actual quotations, by the way. We're going to glorify you and send you home a hero, recalled by the Pentagon for morale and public relations purposes. You'll live like a millionaire. Everyone will lionize you. You'll have parades in your honor and make speeches to raise money for war bonds. A whole world of luxury awaits you once you become our pal. Isn't it lovely?
Now, most of us, despite our distaste for it, would probably accept this offer too because, frankly, it would allow us to walk away alive and still remain citizens of our country. Certainly that's why Yossarian accepts it until:
Yossarian waved goodbye fondly to his new pals and sauntered out onto the balcony corridor, almost bursting into song the instant he was alone. He was home free; he had pulled it off; his act of rebellion had succeeded;he was safe, and he had nothing to be ashamed of to anyone. He started to the staircase with a jaunty and exhilarated air. When Yossarian returned the salute, the private in green fatigues turned suddenly into Nately's whore and lunged at him murderously with a bone-handled kitchen knife that caught him in the side below his upraised arm.
In this topsy-turvy world, Nately's whore, strangely enough, seems to represent Yossarian's conscience. Every time he gets comfortable with a compromise, Nately's whore shows up to haunt him, which, of course, also explains why he walks backward all the time. Rather than returning to the states when he could, Nately stayed and died because he loved the whore who seemed to disdain him until his death. By selling out to Catchcart and his friends, Yossarian would be betraying not only Nately but all those other pilots who secretly wished him well in his attempts to stand up to Cathcart. Of course, in the end, it may well be that it was actually the news that Orr had reached Sweden that inspired Yossarian to turn down Cathcart and flee to Sweden instead. Once he found it really was possible to escape Catch-22, Yossarian seems willing to stand up to Cathcart and Milo because:
When I look up, I see people cashing in. I don't see heaven or saints or angels. I see people cashing in on every decent moral impulse and every human tragedy.
Despite what most of my ex-students thought, despite what the conservative hawks would have you believe , Yossarian, like Atticus in To Kill a Mockingbird, is a true hero (if a fictional character can ever be a "true" anything) because he stands up for what he knows is right even though he knows he cannot win. In a postmodern world, perhaps that is all we can hope for. If we settle for less, we are victims of the most insidious Catch-22 of all, the Catch- 22 that exists in our minds.