Throughout most of his life, Regulus had never had much of a choice in anything. As a child he had to wear what his mother bought for him. He had to eat whatever the house-elves put in front of him, regardless of whether he actually liked it or not. He had to associate with the people his father chose. He had to act exactly how his parents said he should. He didn't want to do any of those things, but he knew the consequences of ignoring his parents.
It didn't get much better when he started Hogwarts. His parents made it perfectly clear that any house other than Slytherin wasn't acceptable. He wasn't allowed to join Sirius in Gryffindor. He wasn't allowed to join Ravenclaw where the Hat said he should go. Slytherin was the only choice. It wasn't a choice at all. His father told him who to make friends with, who to avoid, who to impress. His mother told him what subjects to carry on to OWL level and then NEWT level. He didn't want to do what they said, but he knew he couldn't say no.
The only choice he got to make at Hogwarts was to join the Quidditch team. His parents disapproved. His father said it would distract him from more important things. His mother said he needed to focus on his education. He managed to convince them by saying there were a lot of 'important' people on the Quidditch team that he needed to keep on his side and that he needed a way to relieve stress after studying so hard. In the grand scheme of things it wasn't that important, but to Regulus it felt like a massive victory.
After he graduated, his father told him he was to join the Dark Lord. Like so many times before his father made it clear, without actually saying the words, that he didn't have a choice in the matter. He was to become a Death Eater. That was that. Once he joined, any vague notion of having a choice in anything anymore was stripped from him. He had to do as he was told, regardless of right or wrong, regardless of his wants and desires. He didn't want to do any of it, but he knew what would happen if he defied the Dark Lord.
Choice wasn't a word Regulus was allowed to have in his vocabulary.
Until one day it was. Until one day he'd had enough and said to hell with everyone else, he was going to do what he wanted for a change. So he made the biggest choice he would ever make. He betrayed the Dark Lord. He would pay for it with his life, but he would never regret doing what he did. For once he didn't feel trapped. For once he felt in control of his life. For once he felt free.