Why does Vonnegut think that ending the book with Edgar Derby's death would be ironic?

Vonnegut thinks that climactic end of the book with Derby's shooting would be ironical since Elgar was a kind and just man who did not deserved to die.

Explanation:

The Slaughterhouse-Five storyline revolves amidst world war and its climactic endings. In the story, a strong emphasis has been put on Tralfamadorian philosophy. With the death of each of the phrase "So it goes" appears meaning the relative presence of a person in a moment and absence in another.

Hence, in the story Vonnegut not only dismisses the conventional climactic story structure which would probably end in the shooting of Elgar Derby who was otherwise a kind and just man. He was to receive the punishment for a petty mistake of being caught with a teapot. However, the destiny prevails and "so it goes" meaning that Justice does not prevail in death.