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I love writing from an alternate POV rather than the more common ones used in books. It's interesting to explore a character that is normally viewed from a third-person perspective, instead, showing all the dimensions and unravel their many layers. It's very easy to just throw everybody into a simplified category but it takes a little more effort to consider where that person came from, what their experiences have been, and what drives them. People are vastly more complicated than some writers would have you believe.
There's also something invigorating about sinister characters. It can be quite addictive and fascinating to write about them. It allows my imagination to go to all kinds of dark places that you simply can't explore with primarily law-abiding and 'good' characters. And if you do, it becomes the central theme of the book rather than a component. For example, if a 'good' character does something terrible, it becomes the plot, with the protagonist attempting to understand and justify their behavior. If a 'bad' guy does the same thing, it's not such a shock and just becomes another event in the book.
When you're a writer, shouldn't you be searching for different scenarios, alternate voices, and most importantly, intriguing storylines that aren't following the same, tired format?