Ninjas were formed by the one of the first emperors of Japan after the shoguns lost power to the more centralized Imperial Japanese government. Since the emperor couldn't get the Samurai to do their dirty work, they created the Ninjas to be their own government hit-men. Also, the stereotypical Ninja is quite inaccurate to their historical counterparts. A true ninja mostly dressed in commoner's clothing with a concealed weapon, or used the implements of the trade they are impersonating such as sewing needles, farm implements. more often then not, a true ninja will use the crowd as their cover as they pick their target out of it.
Pirates are different, they steal, run away, and live. A pirate is going to have a much more fun life, although probably more short-lived. Many pirates were unsuccessful in the past.
Me personally: Viking
at their height, The Viking raiders were so feared that the Kings of England and France would set money aside specifically to buy off the Vikings pillagers when they came. Remember the song of your childhood "London bridge is falling down"? You can thank the Vikings for that one. when the King of England didn't pay off the Vikings, they hooked their Longboats up to London Bridge and collapsed it. The largest attack, was the siege of Paris with over 700 Viking longships and 30,000 Viking raiders. Not only were they a force to be reckoned with, They aslo had a very widespread reach, as far east as central Russia and Istanbul, as far west as North America and as far south as Africa and the Mediterranean. Europe needed a swift kick in the *** to get out of the Dark Ages, and the Vikings provided it.
Though I do sympathize with other ninjas