Hitler and most of the other major Nazi figures legitimately believed the Holocaust was necessary for the sake of humanity. Which really shows you just how messed up their ideology was. As for the Inquisition, I have no doubt that many believed they were doing the right thing, though some were certainly motivated by self-interest. As for the thread topic, I think the first response covered two of the most common reasons people do bad things. They believe they're doing the right thing, or they're acting out of their own self-interest.
The funny thing @X Equestris is that even knowing that I still kind of find that hard to accept. As the Psychology link someone shared earlier stated, it is pretty simply, the lack of empathy. People do have it. And to varying degrees.
If you haven't already, I would suggest watching 'Breaking Bad'. Psychology is such a major aspect of that show. You have good people that do good things, bad people that do bad things, and everything in between. I mean look at the main character himself, Water White. At the begining he is a good guy, and he makes a decision to do bad things, in order to do good for his family. By the end he is a bad man that does bad things in order to do 'good' for himself.
Seen it - loved it. It makes you appreciate the fact that there's no textbook good guy/bad guy. It also makes you appreciate that, while there's no excuse for doing bad things, there's always a reason. And you can't get poles further apart than excuse and reason.
That's a good point. Not everyone who does 'bad things' are Hitler-level monsters. Sometimes, even the 'bad act' is not even just to satisfy some self-gain on the perpetrator's part. Sometimes the 'bad person' does 'bad things' because in reality, he/she is scared out of their minds and this is how they cope with it, or maybe they've been shat upon by society so many times that they say, 'Fuck being nice, they want a bad guy, they've got one!' And even then, the worst they would probably allow themselves to descend to is being an arrogant, conceited asshole. It's all a mixed bag. And as @cutecat22 alluded to, even the good guys can go bad for a lot of reasons.
Read the selfish gene and Ivana Chubbuck -The Power Of The Actor. Both these books address your question and rather eloquently i might add. The above comment is a bit on the vague side becz its a big concept compressed in a few lines
If bad people didn't do bad things they wouldn't be bad. So really, why do people do bad things? Because people are greedy, selfish and cruel. They look to fulfill their own wants and desires at the cost to others. Intent or ignorance is no excuse.
I'll borrow from things I'm learning in a dialectical behavioral therapy consultation group. The concept of dialectics varies from field to field, but in psychotherapy it's useful in the following way. Somebody says "that person hurt me." On a scale of opposites you might have hurt on one end and help on the other. You might challenge the person to say "did that person help you in any way?" You can embrace opposites and contradictions and find new truths. Somebody who robs me and injures me has cursed me and blessed me. They cursed me in that all these bad things happened for the next few days because I had no money and had to go into the hospital. But along the way I learned how much support I had from my friends and family, and I learned how to face a new challenge and become more resilient. You could call this simple reframing but it's more complicated because it's the bold embrace of opposites and contradictions, which is more difficult and in my opinion more intellectually advanced. In my WIP I'm trying to paint as clear a picture as possible that it's basically impossible to frame my MC as either a good or bad person. You love some parts of her, you viscerally hate certain parts of her. You feel sorry for the people she hurts and you also understand why she hurts them, and wonder if maybe they deserve it. You have to wrestle with where you stand, and you even find that it's difficult to take a rigid stance. Who knows if I can pull this off as I'm describing it, based on my skill level. But to me that's a more enriching moral depiction of humanity than a simple good vs evil one.
The French have this saying: "to know all is to forgive all." If you investigate the question that started this thread too deeply, you could convince yourself that there are no bad people, just misguided/ignorant/overzealous but well meaning people. Possibly not a great point of view for living a morally upstanding life, but I think it may be the point of view we have to adopt in order to write interesting "bad" characters.
This reminds me of the games created by Rockstar; all the protagonists are portrayed as generally good people who were forced to doing bad things because of corrupted officials, etc. To be honest, though, it's OK to have an evil character who is just doing it for his/her own sick amusement or because of a goal he/she wants to reach. Even I get sick of reading about villains who had tragic pasts that was the reason they were villains.
An example of a somewhat "bad guy" that did everything out of goodwill is the character Kiritsugu from the Fate Zero light novels. He is literally a mass murdering, emotionless killing machine who uses any method possible to kill his target. He even kills his father, mentor ad such. However as the story progresses you find out he does all these immensely evil crimes with the overall goal of word peace. As you find out you begin to massively empathise with him, since he literally sacrifices family and friends multiple times to save the world and even then fails. A bad guy is not the bad guy in their eyes most of the time. Adding flaws and giving them a reason to carry on committing those acts CA help te reader understand why they did those things. It could be a noble goal which ha destroyed the person as shown above, or something as smile as revenge.
Sociopaths are also a thing. Lack of empathy can cause the whole "good guy" thing to not be an option
Question - what is evil? Many people enjoy horse racing - yet a nearby track was recently shut down and more than 70% of the broodmares were sold to slaughter houses ... does that make the sport of horse racing evil? Is a farmer who shoots a coyote evil? - What if the coyote had pups, now is he evil? Evil is a very generic term and really should be struck from writing. What is good to you might be horribly evil to others - farmers cutting down the rainforest are merely looking to make money / survive, yet they are destroying the jungle & displacing countless aboriginal tribes ... are they evil? The question is like asking why is the sky blue? There are countless reasons for someone to be "bad" or "evil". Mental - mental disabilities, psychopaths, etc. Environment - someone grows up knowing no better, sociopaths, etc. Spiritual - they believe this with their very soul; Crusades / witch hunting, etc. Moral - someone is doing something immoral [e.g. corrupt government], etc. Revenge / vengeance - what is revenge / what is vengeance. In a city here - Vancouver I think - a drunk driver was dragged out to the bushes, beaten quite badly and if I remember had a finger or two cut off probably by the relatives of the boy he had killed driving drunk. Who is evil there ... the drunk or those seeking vengeance? Accident - self explanatory. Pick something - just don't make him "evil" because he likes doing it. Sounds corny - sort of like Dr. Evil in Austin Powers.
Psychopaths lack empathy - some schools of thought say they lack emotions period and merely emulate what they see others doing [e.g. a psychopath laughs when everyone else does even if they don't get what is funny]. Ted Bundy was a real psychopath and unlike most sociopaths he wasn't an outcast - that's another defining feature sociopaths are oftentimes society's outcasts while psychopaths can get along in society quite well/normally - but very charismatic. Sociopaths may lack empathy - however, unlike psychopaths their behavior is taught / learnt / copied from what they experience / see and as such in a very extreme situation a sociopath could be redeemed because as like how they are taught to be "bad" they can be taught to be "good". The character of Riddick for example could have been considered a sociopath - all he knew how to handle things was violence & crime - because of how he grew up... but he was a "good" sociopath. The characters that Harris used to form Lecter are perfect of the two types. The person later dubbed Dr. Salazar would be a psychopath - charming individual who if it really was TreviƱo had a he-did-it-cause-he-could attitude. Dykes Simmons would be a sociopath - whose violence was due to the fact he was an outcast [he was said to be an ugly SOB]. Hannibal is a perfect combination of these two extremes - he became violent because of his upbringing but because of his lacking empathy had nothing to "gnaw" at him and could perform like a normal human being.
Bad people do bad things because they believe they are right in doing so. "Evil" and "bad" are very polarized terms. They almost never apply if you look at a situation a certain way. People (and characters) are motivated by their own goals and beliefs, not because they are this overlord that yearns to destroy the world, people don't work this way. At least, they don't normally.