I’m fascinated by corruption. The Godfather shows a sympathetic, promising young man who becomes the ruthless leader of a criminal enterprise. Citizen Kane shows how a clever, idealistic boy transcends his loneliness to change journalism and then yields to temptations that overwhelm him. An eagle scout studies chemistry when racism forces him to abandon his quest for a PhD. Instead, he dedicates himself to leading the fight for Civil Rights, becomes a successful mayor of Washington, D.C. and then gets caught up in drug abuse (Marion Barry).
The essence of corruption is power revealing a character flaw. Power acts as an amplifier, opening up him or her to new or bigger temptations and/or giving the character the opportunity to get away with harmful behaviors. The amplifiers could be physical power or skill (think of star athletes), wealth, charm, authority (as in politics), information (secrets, knowledge), allies, or anything else that could give a character an advantage. Note that any of these might be used for good, for evil, or for both by the character. Also, the power need not be absolute. It can be relative. A ten-year-old bully is not apt to dominate a town, but he can easily dominate those in fourth grade or younger.
As with the opening examples, a whole story may be built around power leading to corruption. Some great tragedies and wonderful villains can emerge when the whole arc is explored by a writer. Once you, as a storyteller, recognize how corruption might transform a major character in the tale you have to tell, you can find those moments, often small decisions, that make the victory of evil inevitable. It is these scenes that can make or break your story. While the larger arc may make a great logline, it is the small compromises that engage readers emotionally.
One of the best choices is getting the readers to sympathize when they probably shouldn’t. For instance, a bullied kid might get justice at last and have readers cheering. Taking that a half step further, tipping it into revenge or an out-of-proportion punishment of the bully is likely to keep the readers on the hero’s side, pulling them along as co-conspirators until things begin to go out of control.
Balancing good choices with small bad choices that add up (or slowly escalate toward evil) provide a seductive pace that can lure readers into deeply problematic situations. But attention must be paid to justifying bad choices, either to balance a grievance or because allowances seem fair (the character is so good, the rules really don’t apply or the vice makes up for noble sacrifices).
Here’s something I’ve found to be most effective. Look for change in power in a scene, where your character (usually the protagonist) is on the winning end. And have the character respond to that win by using the new power in a way that crosses a line — slightly, early in the story and grossly, later in the story. In other words, build slowly so it feels authentic and a little unsettling. Do it right, and you may create as powerful (and corrupt) a character as Walter White in Breaking Bad.