Not that you would be expected to know this, but it wasn't typical for middle-class or wealthy Englishmen to drink at inns like this. Ancient inns were more for the rough-around-the-edges types.
"But ranged as infantry, (5)
Wealthy, educated men in the military were often officers of one sort or another, and infantrymen were lower-class men hoping to distinguish themselves in battle. This was one way to rise in society, but it's also a strong indication of just how much class matters on the battlefield. After all, it's the working class infantry men who die in the greatest numbers—they're the ones on the front lines.
Was out of work—had sold his traps— (15)
The speaker is letting us know that he didn't have much choice but to join the army, because he had no job or money. Sure, he's saying it about the man he killed, but he might as well be sharing his own life story. It's clear this guy was not a born warrior.
You'd treat if met where any bar is, Or help to half-a-crown. (19-20)
The speaker is obviously a generous guy, and probably a pretty good friend. Which makes the fact that he had to kill this other man an even bigger bummer. And the only reason he had to in the first place is that he couldn't afford to stay out of the war. Or at least, that's one way of looking at it.