The overall figurative meaning in this poem is that killer or sniper "dies" when he kills for the first time. It could also be supported that he becomes a "grim reaper," bringing death upon all those he touches. The third stanza is key to understanding this meaning:
And look, has made a man of dust
of a man of flesh. This sorcery
I do. Being damned, I am amused
to see the centre of love diffused
and the wave of love travel into vacancy.
How easy it is to make a ghost.
The once "man of flesh" is the sniper, who is now a man of dust. When the speaker admits to the "sorcery" he does, he explains that his is amused by the killings and "how easy it is to make a ghost." He's lost his humanity in a sense here and will never be able to go back to the way he used to be.
Those without military experience may not know that 'ball' is the standard type of ammunition used in guns/rifles; e.g. 5.56mm Ball.
In stanza one, therefore, the poet could be showing how a child playing with a ball is forced to grow up when faced with these 'balls' on a daily basis. Everything thrown or shot (balls, bullets, shells, grenades) is subject to earth's gravity and will describe some sort of parabola when it is. As a sniper in WWII, the persona will be using a bolt-action rifle, therefore the 'ball' in his closed fist is the bullet calling out to him to use it.
I agree with the previous poster on the overall theme of the poem - how repeated killing dehumanises those that do it. The lack of emotion in the poem when describing the act of killing and its results shows that the persona has become immune to the normal human feelings he should have while doing this.
There is some regret - he sees himself already as 'damned', condemned to hell when he dies, and also sees Death as his closest companion and colleague in what he does - just as a wizard's familiar accompanies the mage everywhere and assists him in the magic he does.
This is one of my favourite poems for use with my senior classes - so simple at first reading, but covering some complex themes and use of language. Hope this helps...