According to the poem, the easiest way to kill a man is to deposit him somewhere in the middle of the twentieth century. This is a very interesting perspective, as the poet may be highlighting the effects of the modern age upon the average man. But, what's special about the twentieth century?
The twentieth century saw many technological advances that greatly benefited humankind; the growth of the Internet led to the development of social and business networks that could accommodate global demands. However, these advances did not come without a cost. Even as people became more connected on a global scale, this interconnection did not lead to an increase in intimacy and social cohesion. The implication is that a man's spirit could die even before his body did.
In the poem, the narrator imagines that there are four other ways to kill a man, but they are all cumbersome. In the first stanza, he begins by describing what can only be the crucifixion of Christ. All the elements of the gospel narrative are present in this stanza: 'a cock that crows,' the vinegar, and a 'man to hammer the nails home.'
In the second stanza, the narrator brings us to the medieval age, where men kill each other with bows and arrows and participate in jousting tournaments. In the third stanza, the narrator sneeringly proclaims that, in the modern age, we can now dispense with 'nobility' and do away with pretensions about killing. He alludes to the poisonous gases that led to excruciating deaths in the trenches during World War One. The 'plague of rats' in the trenches were real and did happen during the war, while the 'round hats made of steel' may have been a possible reference to the metal helmets soldiers wore during WWI.
World War One steel helmets.
Germans and Poison Gas During World War One.
In the fourth stanza, the narrator alludes to the destruction caused by the atomic bombs during World War Two. Notice that killing has now become a more detached action that can be executed from a remote location: with the flip of a switch, whole populations can be annihilated without the messy aftermath of arm-to-arm combat. All that would be needed to develop such weapons would be a 'nation's scientists' and 'several factories.' Basically, new technology has made the increasing ruthlessness of man possible.