This poem is filled with irony. The author refers to several kinds of "warfare."
First he refers to the crucifixion of Christ (plank of wood, cock that crows, and a hill, etc.).
Next, he refers to the killing of knights during the medieval period (length of steel, metal cage he wears, and a castle, etc.).
The third stanza refers to World War I (gas, rats, and a dozen songs), while the fourth stanza refers to World War II (the atomic bomb and Hitler—the psychopath).
Through the entire "timeline" of the poem, the author talks about these "cumbersome" ways of killing, while paradoxically each stanza shows advancements in more sophisticated ways to kill.
However, the pivotal point in the poem is found with the final stanza. It reiterates that all the prior methods listed are cumbersome. This is a surprise, not for the first several stanzas, but it is for the stanza on World War II: when advanced technology had created an atomic bomb that brought about Japan's surrender.
The final irony is the author's message that advancements in technology provide no better way for killing: the best way to kill, he pro ports, is to leave mankind to its own devices. By doing so, men will kill themselves in the way they live during the most advanced age known to man, the twentieth century. In other words, when mankind should have the most answers to avoid war, without any help the human race will "self-destruct."
I would say that this poem is pretty pessimistic about the human race's chances for long term survival. The poet is saying that (in his day) it is so easy to kill people that a person living in his time is pretty much as good as dead.
The poet goes over how killing people has changed since the time of Christ. As he points out, we are getting better and better at killing people. It used to be difficult to do, but now it is getting easier and easier to the point where all it takes is the press of a button.
Because killing people is getting so much easier, the poet seems to fear for the survival of the human race in the long term.