I've enclosed the poem here for you to look at as I go through the different elements of it. If you take a step back from it and look at it as a whole, you'll note that in the beginning of the poem, with the Christ reference, the killing was very personal. There are people there at the site and they are actually touching Jesus. As you move down, the killing gets increasingly impersonal, to the point at the end where one is able to kill a lot more people with no hands on. The killer needn't look the person in the eye as he is doing it. The impersonal nature of it allows someone to kill a lot more easily. At the end, however, there is an ironic twist, in which the poet says the easiest way to kill someone is to leave him alone. In that manner, I think he's saying that we are meddlesome by nature and we need to do this to survive ourselves.
There's a link at the bottom that will help you get started with your line by line explication of the poem. Hopefully, this description will give you a head start.
There are many cumbersome ways to kill a man.
You can make him carry a plank of wood
to the top of a hill and nail him to it.
To do this properly you require a crowd of people
wearing sandals, a cock that crows, a cloak
to dissect, a sponge, some vinegar and one
man to hammer the nails home.
Or you can take a length of steel,
shaped and chased in a traditional way,
and attempt to pierce the metal cage he wears.
But for this you need white horses,
English trees, men with bows and arrows,
at least two flags, a prince, and a
castle to hold your banquet in.
Dispensing with nobility, you may, if the wind
allows, blow gas at him. But then you need
a mile of mud sliced through with ditches,
not to mention black boots, bomb craters,
more mud, a plague of rats, a dozen songs
and some round hats made of steel.
In an age of aeroplanes, you may fly
miles above your victim and dispose of him by
pressing one small switch. All you then
require is an ocean to separate you, two
systems of government, a nation's scientists,
several factories, a psychopath and
land that no-one needs for several years.
These are, as I began, cumbersome ways to kill a man.
Simpler, direct, and much more neat is to see
that he is living somewhere in the middle
of the twentieth century, and leave him there.