You can see this speaker in one of two ways: he's a bumbling oaf who doesn't get the magnitude of his actions, or he's a wounded, shell-shocked veteran of a terrible war.
Of course, he's probably a bit of both.
Bumbling Oaf
This guy stumbles over his words (which all those em dashes go to show), and can't seem to say anything intelligent about the fact that he ended another man's life, except that he shot him. Dead. While he manages the ever so keen insight that he and the man he killed could totally have been bros, he can't muster any deep thoughts beyond that. It's clear that the world is just too big for this guy's small mind.
Wounded, Shell-Shocked Vet
There's a reason this guy can't find his words—or his footing. He's been deeply affected by the violence he saw, and he's torturing himself with the idea that he and the man he killed could have been best friends, if the circumstances had been different. We're betting he has all kinds of deep thoughts and feelings about this whole big mess, but he's too wounded to hash it out. Next stop, therapy.
A Bit of Both and Then Some
To be honest, neither one of these options is very satisfying. We can't write this guy off as an idiot of few words and even fewer insights. It's clear from his struggles in the third and fourth stanzas that he's doing his best. And it's also clear from the last stanza that he's a genuinely good guy. In fact, he's the kind of guy who will buy you a round, as long as you're his fellow.
But it's also clear that these circumstances are a bit beyond him. He's just one average Joe in the big mess called war, and he can't quite see the big picture. But that's not really any fault of his. At the end of the day, he's one of thousands of guys who are forced, by orders, by chance, by war to kill other men. There's a cruelty to that fact that's not lost on Hardy.
That's probably why he makes this speaker an infantryman, and not some high falutin' general or mustachioed politician. Their words are too grand, and they'd probably find it easy to rationalize human deaths for the greater good. But this guy who spends his days in bars? Well he just doesn't buy it—and why should he?