Not everyone thinks like you, not everyone has the same experiences and viewpoints as you. There are people all over this country that continue to enlist in and support the military, and view it and the government as forces for good. Obviously he knew there was some controversy, but so what? That doesn't mean he's going to make the same conclusions as you.
You're arguing from a certain viewpoint, and being blind to the fact that people can come from different places that are entirely foreign to you. Maybe he was brainwashed, or willfully ignorant, or something else; whatever the case may be, it's possible for people to come to realizations that they were unable to see before.
A parallel is useful. My father joined the Junior ROTC in high school, and the ROTC in college. He rose eventually to the rank of second lieutenant in the US Army Reserve. He started taking some of his college time to do charity work in Mexico and eventually ended up helping build houses in a Native American community that had finished an uprising against the government. He returned to file for conscientious objector status only to discover he had been promoted to First Lieutenant. Given that Vietnam was escallating, they refused his request for a general discharge and he refiled. They called him up for active duty and he refused. They sent him to to the stockade and he set himself up as the go-to person for helping people apply for conscientious objector status. His request for a general discharge was rejected again, on the basis that he was too honorable for it. Six months later (more time in the stockade, I think totalling around a year), they reached a deal where he would teach a driving course and be honorably discharged. The course was done on film, but by the time they finished it, they discovered that Lt. Travers had made such a name for himself in the stockade in terms of helping people apply for discharges that they couldn't use it. At the end of the day he was honorably discharged and the court martial called off.
It's one thing to watch movies or think about things. It is another thing to see the results of war with one's own eyes.
You're arguing from a certain viewpoint, and being blind to the fact that people can come from different places that are entirely foreign to you. Maybe he was brainwashed, or willfully ignorant, or something else; whatever the case may be, it's possible for people to come to realizations that they were unable to see before.