I know exactly what they mean by 'darker side'. The first time I encountered Satan in my sleep, he was posed as a different version of me. A better version of myself (on the outside), stronger, smarter, better looking, more charismatic, etc. But a while after that, I learned how to spot the irregularities and force him to leave after some time. I had never fully understood why he portrayed me, until some dreams later down the road.

But I once tried lucid dreaming before, and yeah like you mentioned it's like playing with a 'darker' you. At first it's great, you can do whatever you want, etc, etc. but I had this gut urge telling me to stop lucid dreaming, so I went along with it and I'd get these awful nightmares where my my mind would literally split (I have no other way to explain it) and my thoughts were divided. One of course, being a good me, and the other being a bad me. I literally thought for a while I was developing a split personality and it scared the hell out of me, the other half basically went against all the morals I have without question and it made me wonder if I was really capable of being like that in the real world as well. Again, it's really hard to explain. And it makes me wonder if people with split personalities go through the same thing. I'd have nightmares occasionally, but they never frighten me anymore (still don't). At the end of the dream, I'd discover the real monster was myself and whatever else terrorized me was purely a metaphor to symbolize something much more frightening has taken over, and not even it, could match my own capabilities.