Why wasn't Johnny upset about dying?
Johnny tells Ponyboy that those kids lives were worth more than his. He is fully aware, as he has always been, that his home life is strangling him. He is timid, and unable to stand up to his mother. He is aware in his current condition he will be even more submissive than he ever has. Death is his escape. His final words to Ponyboy help to reveal his attitude. He tells Ponyboy to "stay gold." He recognizes that Ponyboy has hope and idealism, and most of all, has a future and possibilities ahead of him. He believes Ponyboy's life is worth more than his own. He is happy now to escape the pain that has been his own life, and most especially because he saved the lives of the kids and Ponyboy (pushing him out of the way of the beam).
I think Johnny is finally at peace with himself. For the first time in his life, he feels as if he has done a positive work. Running into the burning chuch to save the trapped children made him into a local hero, but more importantly, this deed helped Johnny realize that he was a good person. His mistakes were now forgiven, and as far as he is concerned, death is a fair trade. He is content with dying because he knows that his life now has meaning.
He died saving people, so he died in honor.