I'm relatively certain I could get away with killing my immediate family, not so much in the sense that law enforcement wouldn't be able to figure out who did it, but in the sense that I could be long gone before doing anything about it. So long as I had an ounce of planning, and something that could kill quietly, I could be on my way out of the country with enough cash to tide me over for a while. It would take three or four days, maybe even a week before anyone even suspected anything, and possibly longer before police got involved.
I never would of course because I like my famil and am not a psychopath, but these are the kinds of things a fiction writer thinks about. Stephen King highlights the "what if' statement as the inspiration of many novels. This is how writers tend to think. What if a man wanted to die, and it was a good thing (Arnett Tanner Wants to Die). What if I tried to get away with murder? (No novels yet, but we'll see.)
Making things up is not as easy as it sounds. There has to be some sort of natural progression to a story. Taking a single "what if" and building around it can help a plot develop more realistically than the most finely tuned plot outlines. You "what if" can define the key motivation of a character, can be the very essence around which your novel is bound, or it can merely be a starting point, a beginning to a story that you follow until it runs its course. (Which is what mine tend to be.)