First off, the comic book and TV ninja may be amusing, but they have almost no basis on real ninjas. They are often portrayed as a sneaky, cowardly assassin in black who forgets all concepts of right and wrong in order to carry out his plans. This idea often comes from sport martial artists or people who know little about martial arts overall, (not by any means all of them, thats just the area most of it comes from) because many of them are under the impression that there are "rules" for combat since their fighting style is designed to fit into those "rules"(boxing, TKD, kickboxing, etc). They arent willing to do anything deceptive or "dirty" to win a fight, because that would be "cheating".

That is fine for competition fights, but it is not a good idea for real combat.

There are two reasons why.

1: Even in competition fights, the goal is to strike or score before your opponent knows you are attacking. The basis of any fighting sport and most other sports is deception. Examples:the curveball in baseball, camouflage in deer hunting and paintball, rolling a sidekick or roundhouse out of a front kick chamber in TKD and MMA, etc.

2:An attacker will not be attacking according to a set of rules, and will often have something in addition to that to give himself an advantage, such as a gun, knife, greater numbers, high endurance due to drugs, etc. You must use any advantage available to survive.

A "win" in a real fight is when you go home unhurt, not when you beat the other guy. That is one way to win a real fight, but not by any means the only one, and it is often the most dangerous.

Ninjutsu started as a way for peasant farmers in Japan to survive attacks from samurai and feudal lords. They legally had no right to defend themselves, so they were forced to either give up everything, or simply ignore the oppressive laws of the time. That doesnt mean they ran around robbing people and things like that, they just lived on their own and didnt obey laws that helped only the rich samurai class.For this they were branded as outlaws and portrayed as petty thieves (unless they were serving the emperor, in which case they were said to have magical powers and things like that).Keep in mind that those writing Japans history books were those of the samurai class and therefore glorified the samurai. Ninjutsu history was passed down almost entirely by speech until recently, so there are very few "ninja record books" from the time period itself.