"I trust everyone, it's the devil inside them I don't trust." - John Bridger

Everyone has a dark side. I'm not just talking about your character, I'm also talking about your side characters. I know I keep harping on this, but a character is only as good as their support. So how do you crush a character? Take that support away.

What are the three things your character fears most? 

These are the things you hurt them with. Pain is only as effective as the suffering it causes. Have a trusted ally betray your character in the worst way possible. Maybe it's by confirming their idea that they are worthless, or maybe it's a full out betrayal where the ally never cared about the protagonist at all. 

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Say you have a table, the table top is your protagonist, they have three key friends/family members that are their support system. L1, L2, and L3. Now lets say that L2 and your protagonist were recently in a fight, and are not speaking. So that leg is now shorter then it was before. Then L3 utterly and completely betrays the protagonist, and the table falls. 

At some point in your story your protagonist must break. Whether they give up, lash out, or shut down is up to you. But one way or another they break, and after the trauma they are left to deal with the collateral damage of their failure. 

Heroes are not perfect. They bleed when they fall down, the courageous are afraid, and the optimists lose hope. But they survive, thats what makes them heroes. They overcome the insurmountable odds that we place in their way. But as F. Scot Fitzgerald warns:

"Show me a hero and I'll write you a tragedy." 

It's our job as writers to create a tragedy. The stories we write open doors to those who don't believe they can overcome their situation. And no matter what fans may say, no one will read a novel where there is no conflict and the hero has nothing to lose. It's this darkness, this other side of things that is so crucial to a good story. 

When a character loses their support, they lose much of the trust that is so important to their quest. And wether they still have the support of their other friends or not, it is quite likely that they will no longer trust their remaining companions. Wether it's a death and they are coping with their grief, or  betrayal from someone close, heroes often push others away when in pain. 

Notice I say often, not always. Tragedies are a stylistic technique and can be administered in very different ways, or not at all. It's up to you. YOU are the author. Not your editor, or the publishing agency, not even your friend who reads everything over. In the end the final call is up to you. 

The problem with writing a partial tragedy (I say partial because not everyone has to end up unhappy) is that after the protagonist hits rock bottom, they have to gather the strength to climb back up. But that struggle changes people, it brings out a side that even you may not know your character has. The 'devil' inside doesn't always surface in the way you'd imagine. 

Every character will react differently to destabilization.  Some will lash out, others will internalize anger and despair. Either option creates a cascade of consequences that your protagonist will have to deal with. This 'Devil' inside, though detrimental to a character, is an amazing plot device.