That's really strange. I haven't seen that before. You could decline the quest and get it again and see if you can solve it then. It might reset you back to 0 killed though.
At worst you could start a new game with that character and then skip all of the story quests up to that one again.
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, & Din's Legacy