One question, when you're a hardcore 'ninja' and everything, how are you actually gonna advertise your trade? Put an ad in the local paper?
You know, 'contract killer needs work' sorta thing?
And also, if you genuinely want to learn how to kill things, join the Marines. Or a pest control company, they get to kill lots of things. Mostly Rats and Cockroaches, but you gotta start somewhere. Most serial killers started with animals you know.
Also note that 99.9% of people who say there were or are spys, assassins, SAS comandos etc are making it all up, usually for attention. Have that in mind when you're looking for people to teach you the ancient ways of the warrior, whatever you do...