There's something so devastating to me about the Plinth family dynamics. When Strabo Plinth decided to betray his district, the only home he had ever known, he did it for his family. He knew that the districts couldn't win the War, and he knew that the Capitol would punish the districts for their uprising, making life in the districts ten times harder. So he allowed himself to become a traitor. He knowingly caused the deaths of thousands, maybe millions of his fellow district citizens, all so that his wife and child could live better lives.
But it doesn't work. She's barely seen in the movie, but in the book, Mrs. Plinth can't function in the Capitol. She has more money than she could ever even begin to spend, but she's miserable. Other than her son and her husband, she has no one to rely on in the Capitol, and almost all of her friends and family in District 2 have cut contact with her. She doesn't know how to make peace with life in the Capitol.
Sejanus is even worse. While his mother is sad and homesick, Sejanus is angry. He's forced to attend school with people who think of him as inherently inferior for being from the districts. His only friend is Coriolanus, who, on any given day, feels anything from indifference to jealousy to disdain towards Sejanus.
It all boils over during the Tenth Games. Strabo notices how much Sejanus hates the Capitol, and how badly he wants to return to his District, and he assigns Sejanus to Marcus, the male tributes from District 2--one of Sejanus' childhood schoolmates.
Just like everything else Strabo does for Sejanus, it's an attempt to help. He remembers the Dark Days, he knows how terrible things can be in the districts, and he knows how hard things would be for Sejanus specifically if he tried to return. But, just like everything Strabo does for Sejanus, it backfires. The guilt of his involvement in the Games and, by association, Marcus' death, leads him to his decision to enlist as a peacekeeper and help the rebels in District 12, which he ends up being publicly executed for.
If Strabo hasn't uprooted his family and moved them to the Capitol, Sejanus might have been able to live a full life in District 2, and Mrs. Plinth wouldn't have lost her only child (and only friend).
But he fell for the Capitol propaganda.
