Very simply, Holden is growing disenchanted with what he feels is a world of "phonies." Holden is getting older and more aware of the "real world." Chapman felt Lennon was a phony who sang about peace and love, but lived in great wealth above the fray. The same could be said about Reagan, a supposed man of the people. Holden even singles out Hollywood as a source of this great phoniness and Reagan having been a famous actor supports this (Holden's brother works as a writer in Hollywood, so Holden writes him off as a hypocrite). Both Chapman & Hinckley felt justified in what they did, believing they rid the world of hypocritical phonies. This is, of course, a very limited analysis of Holden's complex character.
The text itself; I am a teacher and I teach Catcher.