To be fair, he is in modern world with the police and laws in stuff, not some random fantasy world no more. So there is lots of possibility in terms of social image destruction and whatnot if need be (esp since other dude is famous), as well as rules to follow if he wants to live there properly (something something isekai timeskip means life experience lost so protags would still want to at least partially be able to live that school life he missed out on).
Granted, this also poses the question of how other dude expects to get away with killing/harming protags after they were in the newspaper and also have connections (also to the newspaper company) without being affected himself, so the story really messes up in deciding when modern sensibilities make sense or not.
EDIT:
If we wanna rational more: he def pinned some familiars on the dude so he can react if he ever tries something to his family or smt so there's probably not much risk in letting him go since he also has the video evidence and all to leave it to the police and media (since killing famous person is problematic in the position he's in, and could become a problem later on if he doesn't do it "properly")
This of course can be proven wrong if protag somehow doesn't react to the dragon appearing, because he's supposed to be sensitive to this stuff as well lol.