What is a job in the government like a mercenary or deference contractor?

Perhaps you mean defence contractor.

That's what a vigilante does. There is no job description for a single person in the govt like the one you describe. Agencies do that.

I don't think you know the right people or you wouldn't be here asking.

Contractors, which is a mercenary, work for civilian defense companies. Do you have an elite military or special services background? Apply.

You have to have a talent. An agency, I'm not talking about any specific one, would have to know about you or be able to check you out. They have to recruit you. You won't be "picking up cases" yourself and you won't cold call your way into this.

There's various ways to go about what you want to do, but you will not be paid and you will be subject to arrest and prosecution.

I'm always curious when I see questions "I wanna be a hit man. How do I do it?". I think "Oh, he/she just saw John Wick.".

Here is something you can compare where you are at to. I sent 10,000 rounds a month down range when I worked. i was a nobody. How about you?

The government does not hire mercenaries. They have the Navy Seals for that. If you mean "defense contractor" to talk to Blackwater. Maybe they're hiring.

Read the Fifth Amendment, it tells you that your mercenary/vigilante fantasy is unrealistic, and illegal.