What stood out to me on my first watch was that violence all seemed rather pointless. Barry was Irish and enlisted in a was as an English soldier, and he was willing to kill but he was always ambivalent.
He did the duel thing, as his father had done, and he believed he killed someone and that didn't really seem to faze him. He remained focused on his desires—his cousin and his social standing.
He got in a fight with someone who insulted him and he was made into a mini-hero for that by the other soldiers. He didn't have the wit to 'fight back' verbally and so he fought like an buck, locking horns.
He wasn't fighting for anything at all except, the narrator told us, to be part of high society. But high society involves verbal sparring and he's not equipped for that and always will resort to lowly violence.
Then he was made to fight for the Germans, and he did that, too, worried only about how he'd get out and get on with his life. So he fought wars for two countries he didn't care about or belong to.
Then he married and got what he wished for, but an obstinate high-born little boy offended him...so he got violent. Later on, the older boy did it again (justifiably) and he got violent again, this time in public. He had believed until then that he had passed for a refined and dignified gentleman and be gave up the charade in front of an audience who could no longer pretend he wasn't some dumb animal.
I guess I saw his character growth arc as being when he chose not to shoot his adoptive son and take the consequences, and to return to his lot in life.
So I guess in the end my read of the narrative is that violence isn't useful or noble, whether war or fraternal or domestic, but instead it is a means of expression for people who don't relate to one another and have no language to express themselves verbally.
And perhaps an undegirding demonstration of class divides, as Barry's whole lifelong goal was to be wealthy and significant and even when he scammed his way into it and felt like he was successful, he always knew he was an imposter and, we learn, so did everyone around him, including the little boy he helped to raise.
I was only a little bothered by Lady Barry's depiction. It was interesting that she was comically melancholy. I don't know if I have ever seen that before. But I can't reason at all what drove her decision making process. She definitely didn't seem like a 3-D character to me like the others did. Even the German woman Barry had a tryst with had an understandable motivation. But Lady Barry got the hots for the new Irish guy and chose him over her decrepit old husband (OK, I can follow that), but she totally abandoned her little boy emotionally and continued to fawn over her asshole husband until the end, when the son she abandoned came back to her. Her performance seemed to me like a depressive person, which typically would be a deep-thinking person, but her behaviors came across like those of an intellectually disabled person and I don't get that.