How is it that Borkul the Beast is a Forsworn?

Solution 1:

I don't think he's actually a forsworn... or at least, he wasn't until he got into Cidhna Mine. If you talk to him about his prison time, he answers:

Guards brought me in about 12 years ago. Was running a good group of bandits up until then. But these Forsworn. They're nothing like the men I cobbled together. They're real killers

The way he says they instead of us, suggest that he's not one of them... he's just Madanach's bodyguard.

If you release the prisoners, he will join them in Druadach Redoubt, to start his life anew as a part of the forsworn, according to UESP