There are multiple reasons why dark characters are popular.
- Often times heroes are portrayed as very white and idealistic. That makes them too good and unrelatable. Shades of grey make a person more real. (Arjun/Bheema/Karna are more popular than the likes of Yudhisthira, cops who bend the law for justice are more relatable than the upright ones, iconic Vijay Dinanath Chauhan is essentially a good man who does good things in a dark deplorable manner)
- Psychologically dark characters are way more intriguing than the good characters. They provoke our brain cells and makes us wonder what makes them that way. (Draco is more interesting than Harry. The story of a pickpocket is more interesting than the story of an ordinary teacher or office clerk, every movie on serial killers and gangsters - there are more movies on them than revolutionary world changers)
- Civilization has conditioned humans to seek and tame wildness. Whether it is a love interest who does it or a mentor - a dark character is often seen as a beast to be tamed. It stokes human ego of taming wilderness. (Darryl's softness for Beth and Carol in Walking Dead, Jai and Veeru's loyalty to Thakur, the loyalty to Rana Pratap Singh in Karma)
- The archaic fallacious notion that bad boys make better lovers or romanticization of taming the bad boy (a whole slew of Salman films in the late nineties, Ghulam)
- They manifest a darkness we socially cannot. We vicariously express our inner darkness through them. (50 shades of grey is a prime example, BDSM fascinates everyone, but we are too afraid of inflicting or receiving pain, even for people into it - it is embarrassing to admit, hence such stories fulfill our dark fantasies)
- Sometimes its just an actor, character or script that stands out. (SRK in Darr is one example, the script calls for a villainous character and in the Hollywood original the character is the villain you dislike, SRK portrayed it in such a nuanced manner that even the sane of us were rooting for a psychotic dangerous stalker instead of the actually good boyfriend)
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