Because I truly believe that the heart of a criminal beats deep within us all, and the more I think and explore and write about crime, the closer I get to understanding why each of us chooses to commit a crime or not. You'll never persuade me that anyone exists who isn't tempted, from time to time.
Also, because in real life, just like in mystery novels, you don't really know who it is until the end. We had a workplace theft problem at a theatre I once worked at, and who it was, was the last person I expected it to be. If you worked at the Serramonte Six in Daly City, that's right, I know all about taking Red Vines and hot dogs to Pacifica for beach parties. Don't worry, the statute of limitations has run out. The theatre is long gone.
Finally, the connection between financial fraud and murder fascinates me. Like murder, financial fraud leaves a more or less permanent record. For example, one way to embezzle money from a company is to disguise stolen funds as expenses. But that throws the income-to-expense ratio out of whack, and that bad ratio is filed with the company tax records. Stealing cash is like trying to hide a body. And some of the rationalizations are even the same:
"He deserved it."
"She pushed me until I had to."
"If I couldn't have it/him/her, no one would!"
Dr. Lizardo/Lord John Whorfin, from the movie Buckaroo Banzai, famously said, "Character is what you are in the dark."
I'm just trying to find out what we are.