User blog comment:Knightofthecart/Queen's behaviour in Arthur's Bane/@comment-4243037-20121007104235

She could've recognized the difficult position Sefa was in, wanting to help her father, and exiled her instead. It's not like her existence was a threat to Camelot in any way, the only reason she was able to do harm was because she'd gained Gwen's trust. Exile would've ended the threat just as surely as Sefa's death. Or she could've thought about the kind of message it would send to let Sefa go as opposed to killing her.

Her decision fits with the show's theme of creating your own enemies though, so I wasn't all that surprised by it.