What if the technician resigns and is threatened by the boss?

It's hard to say if it's horrible. Nowadays, many companies run by individuals are not clean, and many are run by underworld people, so it is necessary for you to consider whether what they say is true or not. I think you'd better not look for their competitors. After all, you know a lot about the company, especially some of their technologies. If you go, it is really not safe. What's the use of making more money? Put your life in it. . .

Why not consider developing in other cities? Or simply change careers.

There is another way for you to fight them, and this can't guarantee your safety. Now the society is very chaotic, and sometimes it is very troublesome to offend some people. It may not work if you call the police.