Most of Wilfred Owen's poetry deals with World War I and the horrors of modern warfare. There's a lot of death and killing, and in general Owen paints a pretty bleak picture. "Arms and the Boy" is no exception. Nearly every line in this poem is about death and killing and the different methods that civilization has developed for achieving the goals of warfare.
Despite all the horrors of war, however, Owen suggests, as he often does, that the majority of those forced to fight in wars are innocent, both in the sense of being young and inexperienced but also in the sense of being not guilty (they are not cold-blooded killers by nature).