My good friend Limpid showed me a passage from her psychology textbook (Psychology Core Concepts, 7th edition by Philip G. Zimbardo, Robert L. Johnson and Vivian McCann) of how good people turn bad that just blew my mind, so I have to share it with all of you (but that's coming at the end of this how-to. First I'll give a few examples before providing this proposed mechanism).
I hope this is common sense, but this how-to is NOT inspiration for how you can become a psychotic dictator and commit crimes against humanity. These are ideas to explore in your stories, with your antagonists and even with your protagonists and supporting characters. This is for ideas of how to realistically make your good characters go bad or how your villains are the evil SOBs they are today.
The man who came up with this theory, Philip Zimbardo (who's one of the authors of the aforementioned textbook) is the head researcher behind the Stanford Prison Experiment. It was a research study conducted at Stanford University back in 1971 and funded for by the US Office of Naval Research. This study took male college students and assigned them to be either prisoners or guards. The "prisoners" were actually arrested and went through the entire arrest procedure. They were subjected to strip searches and given prison outfits. The guards were instructed to only call the prisoners by their assigned number. The prisoners performed degrading tasks such as cleaning the toilets with their bare hands. When punished, they'd get their mattress taken away and would have to sleep on the floor, etc.
The results were mindblowing. Both the guards and prisoners took their roles much too deeply and seriously. Some of the guards started showing genuine sadistic tendencies, subjecting the prisoners to psychological torture. The prisoners were driven to their emotional limits, some having legitimate mental breakdowns. I won't go into details of the torture and abuse they were subjected to in this how-to, but you can read about it online to understand the full scope of this study.
There are criticisms as to what you can take away from this experiment because of the subjective rather than objective nature of the study, the lack of a control group, and the theory that these students were simply "role-playing" than actually internalizing their roles, but I still believe this study is a significant piece of evidence suggesting the influence of environmental/situational effects on human behavior (again, take this entire study with a grain of salt. The results are fully up for your interpretation). You can read more about this experiment and see the actual study description given to the participants here: http://www.prisonexp.org/links.htm (also linked to in the EXTERNAL LINK).
A prominent real-life example is a prison headed by the US overseas (I won't say the name because I don't want you guys looking it up. Just reading the Wikipedia entry on it really messed me up. It took me forever to get to sleep last night because I was so traumatized by it. That's how disturbing and disgusting and horrendous the torture was. If you know what I'm talking about, please don't mention the name or in the comments, either.) What happened was American soldiers subjected these prisoners to the most inhumane physical, sexual, and psychological abuse and torture. Here's an example of inhumane behavior from this atrocious event that really stuck with me:
They took some prisoners, stripped them naked, and piled them on top of each other in a pyramid. There's a photograph of two US soldiers--a male and a female--posing behind this grinning and giving a thumbs-up to the camera. This is a VERY tame act compared to some of the things I just read about happening there, so it's a solid example of the evil in the world and shows that (seemingly) good people have treated other human beings like that. I have always been an advocate that good and evil are just socially constructed ideas, but something like this is just... I don't even think even "evil" can begin to cover it.
And now let's wind down a little with a less disturbing but still profoundly mind-boggling experiment, the Milgram Experiment conducted at Yale University in 1961. The purpose of the study was "to test how much pain an ordinary citizen would inflict on another person simply because he was ordered to by an experimental scientist." They had subjects come in to be a "Teacher" and a "Learner". Unknown to the subjects, the "Learners" were actors, so really, all the subjects just took the role of Teacher.
The Experimenter (the one conducting the study) told the Teachers that the Learners would be asked questions. For every wrong answer, the Teacher must give the Learners an electric shock. At first, the shocks were only 15 volts, so small you probably can't even feel it. But with each wrong answer, the voltage would go up, maxing out at 450 volts, which can be lethal. The shock machine also had warning labels at a certain voltage and then XXX at 450 volts, so the Teachers were well aware that this was dangerous and life-threatening. The Teachers would give the shocks, and the Learners (they were just actors, remember, so they weren't actually getting shocked) would start screaming and begging the Teachers to stop. They were instructed to shout "I have a heart condition!" But the Experimenter continued to encourage the Teachers to give the shocks.
It was theorized only 1% of subjects would go that far because only 1% of the population are sadists. What actually happened was that SIXTY-FIVE PERCENT of the subjects went to the 450-volt max. You can see the implications of this.
Here's some great insight from my friend Limpid: "At one point (above 300 volts I think) the ‘Learner’ would just scream and then go completely silent, refusing to answer. The experimenter would tell ‘Teachers’ to treat silence as an incorrect answer and go on, but of course, two-thirds went on despite the ‘Learner’’s dangerous silence. It’s also important to note that many of the [Teachers] voiced objections, concerns, became angry with the experimenter and so on. But the important point is despite the moral conflict, they didn’t disobey the authority figure even though it was just an experimenter (who was actually a high school biology teacher and not Milgram himself)."
There seems to be a lot of validity to Milgram's experiment because it's been reproduced many times. More insight from Limpid:
"Milgram also conducted 19 different variations of the above study, varying one factor at a time. The percentage of those who obeyed went from less than 10% (‘Learner’ demanding to be shocked; having two authority figures give contradictory commands) to over 90% (instructing an assistant to deliver shocks/having a peer administer shocks). The percentage of obedience was also close to 10% when two peers were shown to rebel.
In fact, Milgram’s experiment was staged in 2010 by a French television show (http://www.psychologytoday.com/blog/homo-consumericus/201103/the-milgram-experiment-french-game-show). It allowed a live audience to vote whether or not to shock the victim in distress – and guess what – an overwhelming majority of 80% obeyed and went on to maximum shock despite the man’s screams of agony and pleas to stop, egged on by a glamorous presenter."
And now the moment you've all been waiting for, here is the mechanism of how good people can turn so evil, as proposed by Psychology Core Concepts, 7th edition by Philip G. Zimbardo, Robert L. Johnson and Vivian McCann:
How to get good people to harm others:
-Provide people with an ideology to justify beliefs for actions.
-Make people take a small first step toward a harmful act with a minor, trivial action and then gradually increase those small actions.
-Slowly transform a once-compassionate leader into a dictatorial figure.
-Provide people with vague and ever-changing rules.
-Relabel the situation's actors and their actions to legitimize the ideology.
-Provide people with social models of compliance.
-Allow verbal dissent but only if people continue to comply behaviourally with others.
-Encourage dehumanising the victim.
-Diffuse responsibility.
-Make exiting the situation difficult.
And how to get out of this cycle of evil, quoted from Limpid: "if even one person rebels, the pressure to conform with the group sharply decreases and many, many, many more people will be able to resist conformity. Also, note that the opposite of the good-to-evil list could be used for encouraging heroic behaviour."
Chew on this for a while. See how all these points can make a perfectly respectable person turn into a devil-like being. Hopefully you can make parallels between this and your characters--be it protagonist, antagonist, hero, villain, or supporting characters. This might be a mechanism to consider when making your villain commit unspeakable acts against humanity or even just start gearing them on the path toward this until they realize they're turning evil and fork off onto another road. Your hero might be the person giving those 15-volt shocks (metaphorically) and start to raise the voltage, until they snap to their senses and change their ways.