[concept theory] Emotions & Magic

Evidence 1
Starlight Glimmer mentions how her magic is influenced by her emotions by stating the stronger those emotions are, the more powerful it becomes. In this scenario, it is questionable whether emotions - especially primal ones — would overwhelm them or make them susceptible to the delving into and/or utilizing dark magic.

Evidence 2
On a note of subjective irrelevance: What exactly are the Princesses' alter egos? Yes, we are aware of who they are, have a segmented generic understanding of how they came to be, and what they do and serve for. What is unknown is what they are themselves. Luna transforms into Nightmare Moon based off of her frustrations and growing hatred towards Celestia. It was Luna's jealousy that caused her to raise the Moon over the Sun, causing a solar eclipse, during the annual Summer Sun Celebration. This apparently had an effect on her body and mind as she was engulfed in a black ichor or smoke, resurfacing or falling out as Nightmare Moon, who has a different personality and mind than Luna. This too can be said about Celestia's alter ego, Daybreaker. It seems both Daybreaker and Nightmare Moon, maybe even Midnight Sparkle for that matter, are the products of primal emotions and dark magic. If not dark magic, than the magic seen during Luna's transformation is simply an overwhelming onrush of advanced magic.
With these noted, I wonder if the Princesses' alter egos are simply manifestations of the darker side of the psyche. The presumption may be likely as almost all alter egos had made mention of how it is impossible to truly rid of them, this being the case of Daybreaker to Celestia and Midnight Sparkle to the human Twilight Sparkle, or Sci-Twi.)

Conclusion
If the presumption were to be somewhat accurate, I believe that every single character has an 'alter ego.' For instance, Pinkamena Diane 'Pinkie' Pie's alternate incarnation is Pinkamena, an entity or incarnation not exactly canon to the series. Nonetheless, it seems corruption is the ultimate key to unleashing and revealing these supposed 'alter egos.'