- Study ninjitsu and become a vigilante superhero at night
- Become a cop
- Volunteer at a homeless shelter or soup kitchen
Your first part-time job as vigilante requires a lot of ninjitsu, and probably very expensive to pursue since you may have to study in Japan and learn the language. However, you could graduate from college first and become an ESL teacher in Japan. While there, you could also become a vigilante against Yakusa. Though, being a gaijin might not protect you from retribution unless you learn the soft art of stealth and avoiding being unmasked when knocking off confirmed Yakuza members.
Also remember that ninjutsu teachers will never introduce you to their ninja clans unless you learn to be more Japanese than they are. It is most likely that such a teacher belongs to a clan that the Japanese government retains secretly to spy on yakuzas. However, it is unlikely that your work will be appreciated in Japan since officially ninjas who kill people are criminals, even when knocking off Yakuza.
Additionally, yakuza do retain ninja clans to do their spying and assassinations if they don't want to be detected. This is mainly because ninja experts leave few clues that point to who did the hit. The really good ones even clean up after slitting the throats of their targets. This includes removal of blood splatters and especially the wearing of gloves. As well, they never use guns, unlike the amateur guns that yakuza traditionally hire when they are fighting other yakuza groups and the government. There will be no witnesses either, because ninja usually kidnap children whose parents they have to kill to train to be ninja assassins, too.
However, don't be a Newport Ninja.
This is why I consider criminals assassinating their rivals to have consistently botched their jobs. I guess they are too cheap to retain ninja assassins and have yet to consider that route anyway. Until then, they are going to get caught eventually.
So why even bother?
As for the second suggestion, I am assuming this is the career for most criminology students. You are not there just to become a criminologist; you are there to be a cop with criminologist skills in forensics and the works. That way, you get a better pay as a cop. As well, you could also become an FBI agent or a criminal profiler (or its equivalent in another country).
It might even help to have ninja skills. It never hurts to have a hobby related to police work. Additionally, you will know in advance if a ninja hit squad wants you dead.
As for the third suggestion, you need to be a liberal to consider to volunteer to work at a homeless shelter or soup kitchen. So your degree is going to be in social work.
It still would be cool to keep the ninja hobby because the tips you get from the homeless could help you turn captured drug dealers over to police and robbing corrupt politicians making money off the suffering of the homeless to share with the homeless.
Usually captured dealers will be phoned in after being knocked out and left in a safe place, usually their homes. Then the vigilante just uses a prepaid cell phone to give details to CrimeStoppers and pick up the $1000 to share with the homeless.
In short, I suggest you become a modern day Robin Hood ninja whose day job is either a cop or national policeman or social worker.
The wrong way to be a ninja: http://baffledspirit.blogspot.ca/2013/02/the-newport-ninja.html