You could have easily gotten away in that situation. All you had to do was flee/quaff, or flee/run away. Onya was the type of person who would rather wait and catch you in quicksand, or in the wilds where it would have been harder for you to get away. But she also liked bringing chaos, so immolating you was her way of throwing a bit of chaos your way. But you were pretty good dude, I don't ever remember killing you. However, Onya didn't actively hunt Nexans, even if they were dwarves, if there were other enemies on. You can thank Allysia, and Solec for that.