Suspend the reality that you would be an evil/despicable person....
Could you devise and pull off a murder--say a person in your social circles (not a prostitute or a homeless person, sorry TNT)--and remain undetected?
I think it would be very difficult and to pull it off would require some amount of dumb luck. It is nearly impossible to cover all your tracks or not leave some clue. And I think most criminals (or theoretical criminals in our cases) aren't as smart as they think. See Josh Powell (no, he never was arrested, but he certainly wasn't undetected).
Detectives only solve a percentage of murders, but, as has been mentioned on CB, that is skewed by the deaths of homeless/prostitutes/immigrants who are "off the grid" and "unnoticed" by society.
So how clever are you, theoretical murderers? I don't think I'd last a week.