The Depth of Being Free

Sirius couldn't be the Secret-keeper. After his brother died the only thing that was on Sirius's mind was killing the Dark Lord, and when all you want is to get out and fight, you can't make a good Secret-Keeper. So they chose Remus, but he had his own reasons for giving up the task. The next option was Peter, and when he became the Secret-Keeper... Well, we know what happened.