Betrayal of Trust
This film is very much one that is about trust. It begins with Redmond trusting his cousin Norah. That she is in love with him just as he is in love with her. But she betrays that trust when offered the chance to marry a captain in the English army who has wealth, land and position in society. This drives Redmond away from his home and to Dublin where he joins the army himself before setting off on course to becoming Barry Lyndon. The saddest betrayal comes from Redmond upon himself as he betrays the fight and the sense of honor and belief in love that he had at the start of the story to which he sells for 500 guineas a year in trade for his release from the Lyndon family. He becomes a mere shadow of his youthful self by the end and it is at the hands of his stepson, Lord Bullingdon. Bullingdon hates his stepfather because of his ill treatment of him during all his years. The boy was betrayed by Redmond as he was given the opportunity to be the boy's father, but instead chose to merely be a man after wealth and status and treated Bullingdon as an inconvenience rather than with love and kindness. It is Redmond's loss of self that leads to Bullingdon taking everything from him.
Father Son
The father son relationship in this film is incredibly important as it affects nearly everything that occurs. The film opens with Redmond's father being shot down in a dual. We know that Redmond has grown up without a father, without a man to teach him what it means to be a man. Thus when Redmond meets the Chevalier he becomes a surrogate father to him, teaching him what he knows which is how to swindle people in order to gain more wealth and status. Thus, when Captain Grogan is killed in battle we see the opportunity Redmond could have had to be fathered by a man that stood for something noble and meaningful. Instead, he chased after shadows and the allure of place in society with a man that deceived in order to gain it. All of this is to say that Redmond's lack of a true father figure made him selfish and care little for the world outside of himself. He destroys his relationship with his stepson and loses his only full blooded son, Bryan. He is left without an heir, no land and with only one leg by the end. And it is this way for him because he chose to become someone that was against the nature of who he was, and potentially because he didn't know who he was at all.
Social Mobility
Redmond's journey from his home to becoming Barry Lyndon reveals what is one of the major themes in the film, which is that of social mobility. Redmond is rejected of love by Norah for not being a man which means he doesn't have property, money nor social standing. Once rejected by Norah Redmond seeks to be with a woman of such high social standing that by marrying her his position in life will forever change. And this opportunity is given after meeting the Chevalier who tutors him in social standing and how to work his way up in ranks through his time gaming with the man. Redmond soon meets Lady Lyndon and finds his way into her company and marries her after her husband's death. But once Redmond finds himself in a place of power he squanders it amassing great debts and creating an enemy in Lord Bullingdon that causes his exile from the Lyndon family.