No, they won't want to kill you, it's just like burn their money. They'll keep harass or even forced, violented if necessary, but they won't kill you (they might accident kill you but that isn't intended). And not only you, they'll also harass your relative.
Police can't protect you 24/7, they might save you one or two times, but no one can tell what might happen. Someday you might vanish without trace and nobody care. Especially if you involved with the underground (like why they must risk their life to protect someone they not even relate). The police know you're obvious lie if you deny borrowed, because the loansharking never come to you without reason, loansharking also don't want to get trouble with obvious crimes. Not mentioned police have their ways to investigate if that's actually serious case.
From the start, no one go to loansharking if they can loan from somewhere else, they might borrow relative first, then the bank, however the bank require some standard to agree. Then when you have no other choice, you go to loansharking, of course loansharking not stupid, they also don't give money to anybody, they must know that person can actually make money (either sell their body, which is illegal business).
You can't squeeze blood from a stone. People who borrow money from loansharks do so because they have no other way to get money, which means that they obviously have no way to pay back their debts.
They can kidnap girls in japan to make money from their bodies, but from a guy you can't really do that.
So you're left shaking a guy down for pocket-change.
Demanding millions of yen? you might as well demand a spaceship and a pony too.
As for the police not being able to protect you 24/7, that's not what they're there for. It's to arrest the people threatening you.
If you can get them on harassment, assault and extortion, then they go to jail and can't come after you at all anymore.
And yes, people like that do absolutely go after people for no reason.
At the end of the day, 'collections' is just demanding money from people who have no actual official business with you (because loanshark busiensses aren't legal and their contracts aren't binding)
It's just extortion with a threat of violence, you can do that with completely uninvolved people.
The common one in japan (from what I know) is bumping into someone on the street and then claiming that they owe you money for 'medical bills'
It's the most boldfaced lie, but people will often pay just to avoid getting beaten up.
So yeah "these obvious gangsters say that I owe them money but i've never seen them before in my life" might actually pass muster.
Of course, that relies on cops being useful and arresting/charging people who threaten dudes in the street.
but japan has a crazy high prosecution rate, so maybe it'd actually work.
If not, your only option is probably to try to fight them.
Even if you pay, they don't have any legal basis for their loan so the can just say that you owe more and more money and foce you to keep paying.
You just proved that you can get money from somewhere, why would they ever let you go now?
If they come after you and get thrown down a flight of stairs, maybe they'll cut their losses and go pick on someone weaker.
Of course, violent criminals usually aren't the sort of people to cut their losses, and japan doesn't really allow lethal force in self-defence so you'd probably just escalate it.
Don't get involved with loansharks, kids.
They'll bleed you dry and then break your kneecaps.
There's a reason people in this position in japan tend to just kill themselves.
