She's a PC. The player has basically been going for a 'reluctant revenge' schtick. A witch who was secretly training the rogue in magic was killed by the PCs, so when the PCs set out on their next adventure, the rogue (who's from the same village) volunteered to go along. She's been trying to build up the courage to actually take her revenge, though her efforts so far have been half-hearted.
I've been trying to leave her an option to come clean and reform, which could actually work with this party. The party's first adventure, as 13 year-olds, had them capturing a medusa-esque creature who turned creatures to wood with a gaze. She had gone insane from grief, and her blood was the only cure for the lignification. The party, rather than simply killing her, actually had a four hour long in-character debate about what to do.
It really defined the party's style. They've killed amazingly few people in this campaign considering they're 7th level, and they've tried repeatedly to spare people and try to mend fences. Case in point, after the ogre village's first leader led an assault on their village, they killed him, but befriended a surviving shaman and have now managed to integrate themselves into the ogre city.
So the PC might actually survive, if the player decides to keep with the theme of reconciliation. Or she might run and have her resolve hardened, coming back like a thief in the night.