We all have this potentiality and also using knowingly and unknowingly. It's a game of mind where we play with other people's thoughts, feelings, perceptions, etc. to satisfy ourselves by gaining desired goals. For example, we can say that a salesman is very passionate about selling his products to the customer and using dark psychology to manipulate the customers. He is doing this knowingly. On the other hand, a selfish one is playing the game with others' minds to fulfill desires unknowingly. Dark psychology applying can be very harmful cause it can hurt other's feelings deeply. It's also not good to apply on other knowingly.