Just a theory about what pernida actually is. I haven't heard anyone else suggest this yet, so I want to put the idea out there.
I do NOT believe the left arm of the soul king is evil/a quincy/or actually Pernida. I think the actual body of Pernida is a ball of nerves. It may be a logical jump that the quincies captured the left arm of the soul king, and that Pernida used the compulsory to take control of it.
Short story is, I think the left arm is a neutral player, and a captive of Pernida, who may be able to switch between hosts.
This could make for a fun plot point if pernida were to attempt or even succeed to invade Kurotsuchi.