So a lot's been said about how good antagonists need backstories and motivations and whatnot, to the point of it being a general rule of storytelling.
But recently I've seen some people point out that, in real life, that's rarely the case. People who do bad things are often just "selfish and short-sighted", to quote one. They're cruel and even decide to hurt people not because they have some dark history of abuse or loss, but because they just can't be bothered to think about other people's feelings. Or because their own emotions (anger, jealousy, impatience) are so out of control that they block everything else out.
I actually think both viewpoints are valid and real...they're just different categories of evil.
'Dramatic evil' is so common in stories not necessarily because people mistakenly think it's more realistic*, but because it's the kind of evil that most good stories demand.
You want a lasting conflict? Lots of twists and turns; character drama? Antagonists with the potential to become protagonists and vice versa?
You can't build all of that on the kind of impulse evil that causes people to verbally abuse a retail employee. You have to have something more; something for the characters to actively think about and grapple with.
Otherwise your story's gonna end up hella short: if you had a choice between leaving the random McDonald's cashier alone and getting killed by a magical hero with a huge sword, would you stick to your guns?? Most people wouldn't think it was worth it...
*I think it's worth noting that although people don't write and expect dramatic evil necessarily because they think it's more realistic (now that I think about it, I sure as hell don't) many actually do. I think it's partly due to how stories have influenced our thinking and partly due to humanity's innate search for meaning in everything.
It's hard for people to wrap their heads around simple, unprovoked cruelty. After all, "hurt people hurt people", right? No one would put effort into yelling and screaming and being violent without some kind of deep, tragically-flawed reasoning that propels them, right??
To that I would answer that, for many people, yelling and screaming and being violent is actually easier than facing their thoughts and feelings and using judgement and restraint. I mean, ask any 3-year-old. ^^;