"Yes," Regulus answered and he was surprised by how easy the admission came. If anyone, Bella or Sirius, had asked him such a question, he would have fumbled over it. He would have made excuses. He would said something about how it wasn't quite that simple, something about how, when he had done it, he had thought that it was the right thing to do and now there wasn't time for regret it.
After all, even if he was given the opportunity to do things again, he didn't know if they would turn out any differently. He was too bound to his family. He would have never followed his brother. He would always try to do what he thought was for the family. It was a somewhat disheartening thought to think that he was so tightly wound to the Dark Lord - so tightly wound to the fate that he had woven for himself and the death that it led to.
But, at the moment, he was surprised to find how relieved he was to find someone that he could simply just answer. There didn't have to be excuses - because there existed a basic level of understanding. It wasn't Bella or Sirius trying to save him from himself - or each other for that matter.