The war changes Rat Kiley completely; it takes away his innocence as it does with the other men. At first, Rat's quite a happy-go-lucky kind of guy. He doesn't quite fully understand the seriousness of war and the horrors it entails. Neither he nor Curt Lemon have any real comprehension of the dangers they face:
They were just kids; they just didn’t know. A nature hike, they thought, not even a war, so they went off into the shade of some giant trees-quadruple canopy, no sunlight at all-and they were giggling and calling each other yellow mother and playing a silly game they’d invented.
To some extent, Rat's innocence is a metaphor for the general lack of understanding of the Vietnam War among large sections of the American people. Rat and Curt are playing around because they don't really feel engaged with the conflict; this is someone else's war. Nevertheless, they're still deeply involved in this war whether they like it or not and soon Rat Kiley's innocence will be gone forever when Curt's blown to pieces after stepping on a landmine. One minute Rat and Curt are goofing off; the next minute one of them is dead.
Rat is completely changed by the experience. He was always an emotional character, but he was able to channel those emotions into telling stories. But in the aftermath of Curt's death he finds himself overwhelmed with emotions he simply can't handle. We see this when he writes a letter of condolence to Curt's sister. His mind subsequently begins to crack under the strain, and there is a tragic sense of inevitability about his eventual mental collapse. Rat shoots himself in the foot to try and get himself medically discharged. His sad decline is now complete. He has gone from being a take-charge kind of guy, always helping others, to a shell of his former self, desperate and hopeless.