Billy never killed anyone. Most of his unit fell victim to enemy attacks and hostile terrain until only a handful of soldiers remained, then the commander ordered a village to be slaughtered, even after discovering that those were just innocent civilians and not the people they'd been looking for. When Billy tried to prevent this, he was knocked out, as Zacmac said, and framed for the murders afterwards. That's what he was going to be executed for, not killing his unit, so all charges against him were wrong.
But who knows if anyone other than Rebecca is ever going to believe him, so I'm not sure the BSAA would take him. As for HUNK, he has a reputation of always being the lone survivor, not because he loses his unit under tragic circumstances like Chris, but because he willingly sacrifices and abandons them to save his own ass. So no, not a suitable candidate for the BSAA either, especially not as a team leader.
I'd say Helena has potential. She handled herself pretty well in the Tall Oaks outbreak even though it was her first contact with bioterrorism of this kind, and a captain like Chris could surely help her get her emotional outbursts under control (whenever he's not drunk or vengeful himself like in RE6, that is).