Although the discipline involves much use of negative energy and undeath, it also involves the use of positive energy and the ability to combat the undead. It is thus not wholly evil, but a mix of powers.
The Narrow Bridge was first studied by an order of monks who studied the border of life and death. The practitioners of The Narrow Bridge understand that death and life are more closely intertwined than most realize. Those trained in the proper martial arts can cross the narrow bridge that separates life from unlife. While most practitioners of The Narrow Bridge are living, it is not uncommon for undead martial warriors to also dabble in the techniques.
The great masters of the discipline realized the ease at which the discipline could be abused and so were very careful to not teach the discipline to many outsiders. Traditionally, a master would take on a single disciple at a time, one whose ethics and outlooks matched those of the master. However, over time the discipline has slowly become more widely practiced.
The Narrow Bridge was never taught at the Temple of Nine Swords. Due to its association with necromancy and the undead, Reshar disliked the discipline and thus never dedicated much time studying it although he did locate a master who was willing to teach him the basics. In addition to his own dislike, the highly specialized nature of much of the discipline and the reticence of masters to teach it has stopped many martial warriors from taking too much effort to master its techniques.