What is the theme of the poem "Five Ways to Kill a Man" by Edwin Brock?

The wording of the question seems to assume that there is a single theme to this poem.  I don't believe that is the case.  I believe a reader could see several themes emerge from this poem.  I believe there are themes about war, violence, brutality, dehumanization, technology, time, and/or power.  
Of the above themes, I think the dehumanization of mankind theme is the strongest.  I like this theme choice because I feel that it encompasses some of the other themes with it.  
Each of the poem's first four stanzas focuses on a particularly violent event and/or time period.  The first stanza is about the crucifixion of Christ.  Stanza two is about medieval knights fighting and having their armor pierced.  Stanza three is about World War One's trench warfare, and stanza four is about World War Two's atomic bombs.  
The above scenarios are incredibly violent and even barbaric examples of the violence that humans have wreaked upon each other for hundreds of years; however, the author's tone is cold and blunt.  Sympathy, compassion, and aversion to violence are not conveyed by the poem.  The poem doesn't focus too much on human suffering.  The crowd of people watching Christ's death are not described as showing any emotion.  There is no compassion for the man being brutally executed.  The second stanza continues that cold distance.  It ends with a royal banquet that celebrates the destruction and death heaped upon a battlefield.  

But for this you need white horses,English trees, men with bows and arrows,at least two flags, a prince, and acastle to hold your banquet in.

Stanzas three and four almost completely remove humans from the violence.  Both of them are about how easy killing other men has gotten because of improved technology.  Stanza three says that all you have to do to kill is "blow gas at him."  Stanza four makes killing even easier by saying that only a small button needs to be pushed. 

In an age of aeroplanes, you may flymiles above your victim and dispose of him bypressing one small switch.

By the time the final stanza finishes, it's clear that mankind has continually found more and more efficient ways of killing other people.  Our technology hasn't made us kinder and better humans.  It has allowed us to kill from greater and greater distances, which has had the effect of us not seeing our enemy as a fellow human being.  They are dehumanized targets to be eliminated in an efficient manner.  The sarcasm present throughout the poem clearly indicates that the author thinks the increasing dehumanization of mankind is a bad thing.