That's not stupid, but an incredibly smart move, as I believe that it was her intent to help the vigilantes kill Jordan Chase, or to off him herself, and backup would have made either of those ideas impossible.
I dunno, the way Carpenter played it, I got the impression Deb had no idea what she was going to do, and only decided on the spur of the moment, after giving her speech from behind the curtain, to let them go. She's always been impulsive, especially in high-pressure situations, but I don't think she had really fully convinced herself by that point that she would help them. She was still deeply conflicted. Even as she left and let them go, I got the impression she was still doubting whether she was making the right call.