Spare, Elusive, Deadpan
As you'll know the second you look at a page of Slaughterhouse-Five, the book is broken into tiny, teensy sections. Most of these sections are pretty action-packed, so the narrator doesn't leave himself much space to wax lyrical about the events he is relating. The narration in the book is stripped bare of much description, so it's hard to tell what kind of emotional message the narrator is trying to get across.
In fact, the narrator seems kind of grossed out by big displays of emotion. He likes the fact that Billy doesn't cry out loud, and even gives silent crying its own epigraph (see "What's Up With the Epigraph?"). He also describes emotion in pretty unsympathetic terms. When Valencia hears that her husband has been in a plane crash, the narrator says:
Valencia adored Billy. She was crying and yelping so hard as she drove that she missed the correct turnoff from the throughway. (9.1.3)
This is kind of cruel for a number of reasons: (a) we know Billy doesn't feel as much for Valencia as she does for him, and (b) "yelping" is a pretty insulting way to describe a woman sobbing over her husband's potentially fatal injury. The narrator really doesn't appear to think much of emotion, and his descriptions of events reflect this distaste.
Yet, at the same time, the narrator is telling us about the horrors of war. He is describing horrific events that have huge meaning for him personally. Even if he doesn't tell us directly how to feel about these events, he does convey a lot of pathos by appealing to the audience's emotions. We call the tone of Slaughterhouse-Five elusive because there is a lot of emotion in this book... but it's hard to pin down.
The narrator rarely says outright: this sucks. But he shows that it does. For example, in the aftermath of the Dresden firebombing, when Billy, the other POWs, and their German guards crawl through the rubble, the narrator tells us:
American fighter planes came in under the smoke to see if anything was moving. They saw Billy and the rest moving down there. The planes sprayed them with machine-gun bullets, but the bullets missed. Then they saw some other people moving down by the riverside and they shot at them. They hit some of them. So it goes.
The idea was to hasten the end of the war. (8.27.1-2)
There are no adjectives in this passage, nothing like "tragic" or "terrifying" to describe what it would be like for an American POW to be shot at by American fighter pilots trying to kill everything they see. Yet, at the same time, because there is so little emotional description, the overall impact of the scene seems much bigger than you would expect from such a brief passage. The fact that Billy escapes with his life just by random chance, because the bullets miss—and that the "other people moving down by the riverside" aren't so lucky, also by random chance—emphasizes how meaningless life and death seem in this devastated wartime landscape.
The final sentence, that "The idea was to hasten the end of the war," really clinches the deadpan emotional effect of this scene. These pilots may have been deployed to put a decisive end to German resistance, but in reality, on the ground, who are the fighter planes shooting at? American POWs. In just a few words, the narrator manages to convey rage, frustration, and sorrow over the confusion and destructiveness of wartime. This book really does get us every time.