Shaikhul Islaam Ibn Taymiyyah (rahimahullaah) said:

The corruption that arises from fighting during a fitnah is greater than the corruption of the oppressive (actions) of the ruler.  (It is not allowed to seek to stop) the lesser of the two corrupt affairs, and thus (it leads) to the greater one of the two. [Slightly paraphrased: Minhaaj An-Sunnah 4/542]