Have you...? (Short story)

Have you ever walked past a murderer? How would you know if you have? Would you look at their eyes, see the shadows on their souls, or would you notice the clench of their hands at their sides and the tension in their shoulders? How could you tell? 

You have walked across the paths of many of them and you never knew. You never realized the haunted expressions they wore told of nightmares of pulling that trigger again and again. You never realized their fingers had been soaked in the blood of their prey. You couldn't see how they replayed the action in their minds as they gaze into the distance, lost in thought. You wouldn't know if they were protecting themselves from an attack, or had done it in the heat of the moment, or maybe it had been thought of - premeditated - over long nights of fury and hatred. 

How would you know? How would you know that your neighbor, your best friend, your school teacher, your coworker, your boss, even your lover had never killed before? 

Such thoughts keep me awake at night. How would others know, how could they tell? What gives a murderer away? All I know is I'm scared. If others found out, my secret would be revealed, and my fun would be over. Do other killers worry about giving something away as well, or is it just me?