Thank you for ruining my evening with this poem (;.
I would have to vote for "pessimistic" regarding this poem.
In the first four stanzas, the poem describes several ways of killing a man: crucifixion, lancing, gassing, and bombing.
To me, the stanza about bombing is especially frightening, because all it requires is the "pressing [of] one small switch." It is so impersonal that there is no reason to believe that the bomber has any feelings of anger or hatred toward you.
The last stanza, of course, seals the pessimism:
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.
This leaves us with nowhere to escape to (except that we're now in the 21st century, which doesn't seem much better than its predecessor). The poet is saying that the very conditions of life in the twentieth century can (and do) kill people. Noise, pollution, overcrowding, mechanization, mass political movements, rapid communications--all of these are killers.
The poet could have mentioned some of the benefits and conveniences of modern life, but he doesn't. That's because he is a pessimist, at least in this poem.