From Billy's excessively realistic crucified Christ (Chapter 2, Section 19) to the horses with shattered hooves in the rubble of Dresden (Chapter 9, Section 19), much of the suffering in Slaughterhouse-Five explicitly targets innocents. Billy is a foolish, inexperienced boy who is sent to the front lines of a war he does not understand. The injustice of suffering – that it should strike the people who seem least equipped to understand or deal with it – is yet another reason Billy turns to science fiction and Tralfamadore to make himself a new reality. His current, agonizing reality no longer makes any sense to Billy, so he needs another one.
By showing the thoughtless cruelty to animals of even well-meaning characters like Billy, Vonnegut demonstrates that suffering is often an unintended side-effect of war.
Billy does not use the Christian faith as a comfort for his postwar stress and misery because he associates the church with the painful sacrifice of an innocent man, of which Billy has already seen plenty.