A killer normally commits twenty errors, but is only aware of a third of them. Most of them are due to carelessness or inexperience. But there's one type of error that we can consider deliberate. It's a sort of signature. "The devil's dumbest sin is vanity". But where's the fun in being the devil if you can't let anyone else know?
Whenever you’re going after something that belongs to you, anyone who’s depriving you of the right to have it is a criminal. Understand that. Whenever you are going after something that is yours, you are within your legal rights to lay claim to it. And anyone who puts forth any effort to deprive you of that which is yours, is breaking the law, is a criminal.
Indeed, I have found that it is usually in unimportant matters that there is a field for the observation, and for the quick analysis of cause and effect which gives the charm to an investigation. The larger crimes are apt to be the simpler, for the bigger the crime the more obvious, as a rule, is the motive.