In the The Outsiders many factors limit the choices of Ponyboy and his friends, including lack of money and fear of violence. For example, Ponyboy wants to spend more time alone, in quiet contemplation and artistic musings. But that's not exactly an option – he has to stay with his gang for protection. Also, even though Darrel did great in school and sports, and even earned a scholarship, college was outside of his financial range. However, over the course of the novel, Ponyboy starts feeling less trapped as he learns more about the choices that he does have – including choices of violence vs. nonviolence – and begins exercising them.
Although Pony doesn't mention it, his choices on the night of Johnny's death make the tragedy possible, but this still doesn't mean it's his fault.
Darrel's choice to raise his brothers after their parents' death is out of love, not obligation.