The Dart is an unlockable throwing weapon. It cannot kill enemies with a throw but it does allow for executing enemies. Additionally, throwing the dart at an enemy who is holding a weapon grants an extra 500 (+500 per each enemy alerted) points for boldness and the accolade "Big Balls" at the end of the level. It has a unique execution animation similar to Scissors, however it's way longer and doesn't allow for carrying on with the combo even with Zack mask on.
There is a bug that allows to score infinetely high using dart. Leave one enemy alive with unloaded firearm, go close to the wall and throw the dart at the enemy as much as you want.