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So I got the quest to go kill Tenya, and then, after fighting her for a brief bit, she was all "why are you doing this" and told her side of things to me and then I selected that I didn't trust her or the fishermen. After that she turned blue, but I (in keeping with my dialogue choice) attacked her to turn her back red, and she was already at "near death" from our earlier skirmish. However, I kept doing copious amounts of damage to her and she never died; after several hundreds of damage failed to kill her I left and returned to the fishermen and killed them. I've yet to return to Tenya to try killing her again, so I've no idea whether this problem will persist. Perhaps this was caused by her dialogue interrupting our fight right when she was about to die?