The Fatal Flaw
Why is it that whenever a good character turns bad in fiction, he/she suddenly seems stronger?
Simply because he releases or even loses his inner handbrake.
A “good” character is usually caught in an inner struggle:
- there is something that has to be done (like achieving a goal, going on a quest, completing a task, accomplishing a mission). The trigger to this comes from an outside event.
This conflict usually forces the good character not to take the short way: Batman could simply kill the Joker when they first meet. Problem solved (by getting rid of it), fade to black, credits.
In the end, a good character will be able to do, what needs to be done, without “really” leaving his boundaries; he/she might have done so temporarily by bending some rules and thus carefully widening the road a bit, but not by breaking them or leaving the road at all.
Now I have to oversimplify for a moment: A good character that turned bad basically got rid of his boundaries. He has dumped everything that could bring up moral doubts inside of him. For him there is no conflict anymore, no more inner chains that keep him from going berserk. Everything is allowed. Right and wrong do not exist any longer. This is, what makes the bad character seem stronger.
BUT – there is an important hint in your question: you ask, why he/she seems stronger! And this is the point: the bad character never really is stronger.
He/she always has a fatal flaw. And this is because nobody is born on the dark side. There is always a residue of patterns that define a human being.
People always laugh about the habit of the James Bond villains to explain their evil plans instead of killing Bond right away. Well, it is lonely at the top. But humans are social creatures; they are not constructed to be alone. Even a sociopath like Hannibal Lector cannot resist to call Clarice Starling in the end.
If a writer really chose to play that emotional instrument all along a story, he would certainly fail; the audience would laugh about a villain who is weeping about his loneliness. So the writer simply replaces emotional loneliness with intellectual loneliness.
Humans have the desire to share their intellectual superiority (just think about our motives to write on Quora or share a link on Facebook), but for that they need someone who is capable of understanding it. Someone at eye level.
He manages to get close to the villain, and this is by definition what qualifies him for a private lecture on the ingenuity of the plan.
Very often, the bad guys also believe in something – it is simply contradictory to what the hero believes in.
In Goldeneye, it is Bond’s former colleague Alec “006″ Trevelyan who turns bad. Still, he believes in things (in his case, that he has been let down by the government he was willing to give his life for). He wants to take revenge.
In Moonraker, Hugo Drax sincerely believes that the world would be a better place if one got rid of human mankind.
And even if there is no belief in whatsoever: one key attribute of human existence always prevails: selfishness.
24′s Nina Myers is certainly one of the worst guys ever created in fiction. She does not have any moral values at all. Still she has fatal flaw: she wants to live! Everybody wants! It is human nature. The fear of stopping to exist is so overwhelming, that it keeps bad characters from being entirely bad. This is what ultimately keeps Nina from killing her arch enemy Jack when she gets the chance, although she knows that he will make it his personal duty of a lifetime to hunt her down forever. When she understands that she can only get away with Jack’s help, she uncovers her fatal flaw.
In the same series, Jack’s father Philip does not hesitate to kill his other son Graeme (which really felt like jumping the shark even to hardcore fans of the show), but he simply cannot kill his grandson Josh … because Philip wants to “survive” (in this case, survive means to prove that he is not all bad and raise some intellectual understanding in his grandson for it. For that purpose, both he and his grandson have to live, because it will take some more years to achieve that goal).
Good characters overcome their selfishness if necessary. Bad characters never do. That is why they fail.