I write crime (fiction and poetry) because, as engaging as reading it is, I wonder what I (or my characters) would do in the same situations. The curiosity eats at me until I pick up a pen and paper. Yes, that's still most often how I start writing.
If I ever tried to commit a crime off the page, I suspect I'd be caught right away if not in the "just thinking about it" phase. Still, every personality has positive and negative aspects—light and dark sides, as they say—and writers' ability to explore their dark sides on the page can prevent harmful tendencies from manifesting themselves off the page. Writers' conscious exploration results in fewer subconscious tendencies. To know oneself is to have greater self-control.
I also write crime because every paragraph, every sentence, every word seems to have purpose. That's how I like to write anything.