Obviously they were innocent by the very definition of it in the US system: innocent until proven guilty. Obviously if we could prove them guilty we wouldn't need Gitmo, we'd try them in criminal court. We didn't because we had nothing on them.
Also, your "went back to killing" is an assumption. How do you know they were killing before? I would expect that, once released, the people who were illegally kidnapped would seek some action against those that destroyed their lives and held them against their will for years.