I looked into demonic possession for my feelings/symptoms maybe 12 years ago. Even got an exorcism from an Episcopalian priest. It didn't help.

7 years ago I was diagnosed with what was then DDNOS, now probably OSDD. Not full DID because I don't lose time, but close maybe.

I've gotten more comfortable with my "evil" personality -- really evil (mean, heartless, cruel, etc.), is that what you're talking about? For me, I used logic to try to find ways of accepting and understand her awful impulses (rarely acted out but still they were there), angry and vengeful attitudes. They do have a purpose, that's why they are there was my logic.

Eventually it has kind of worked. An expanded "I" can usually accept her for who she is and how she feels. She has good instincts about the shady and the worst in other people which can be helpful to all of "us" sometimes.

It hasn't been easy and an even though I had a T who was a specialist in trauma and dissociation, she wasn't very good and we ended therapy in a rupture without repair. Still, it's better than it was.