Maybe his dark secret could be related to his past trauma before he got his powers. For example, he might have witnessed something truly horrifying that made him the way he is.
The 'Beast Boy Dark Side' in fanfiction can lead to some intense storylines. For example, he might struggle with his identity as he succumbs to his darker instincts. This could cause rifts in his relationships with the other Titans. It also gives room for exploring his internal conflicts, like how he tries to resist the pull of his dark side while still being part of a team that expects him to be the fun - loving Beast Boy.
Well, some interpretations suggest that the Beast's curse and isolation might symbolize inner demons and the struggle with one's own ugliness within. It could also hint at themes of imprisonment and the longing for freedom.
Well, in the 'Beauty and the Beast' dark story, the Beast could be a much more cruel figure at first. He might not just be gruff but actively menacing to Beauty. The enchanted objects in the castle may have darker backstories too. For example, they could be servants who were punished more severely for minor misdeeds. And the relationship between Beauty and the Beast could start with more coercion rather than the slow - building affection we often see.
One of the main themes could be the duality of human nature. The Beast represents the dark side that can be tamed or redeemed. Another theme might be the power of love to transform. Beauty's love gradually changes the Beast.
In fanfiction, Beast Boy's dark past often involves his time before joining the Titans. Maybe his experiences as an experiment, being turned into different animals against his will. This could have led to a lot of psychological trauma for him.
Perhaps one of the main themes could be the struggle between good and evil. The 'beast' in the title might symbolize some form of inner darkness or an external malevolent force that characters have to deal with.