So Japanese defamation laws were designed to facilitate criminals, is that what you're suggesting? There surely are many problems with the political and legal system in Japan, but I think it's beyond ridiculous to assert that the law was created with the intent to facilitate criminals. The most likely explanation is that it's designed to prevent people from exposing every enbarassing details about others. Do note that "embarrassing" doesn't mean "criminal" here.
> In Japan, it’s perfectly legal to record someone
Slightly off-topic, but I don't believe it's that simple. People do have to respect each other's privacy.
> but making the fact that the person is dangerous and not in police custody will get you in trouble, and the person who assaulted you will use the video against you.
Not all video evidence is actual proof that a crime has been committed. If it didn't lead to convictions, then it's not surprising that you'd get in trouble for disemminating it as actual proof. And let me remind you that the Japanese legal system is notorious for its startling high conviction rate.
Another issue I have with your assertion is that video evidence of crime is often disemminated by the Japanese media way before it reaches the courts [1][2]. Some of these are picked directly from social media. Yet no one ever got in trouble for it, contradicting your statement. So far, you've provided zero evidence that what you said is commonplace in Japan.
[1]: https://english.kyodonews.net/news/2019/08/5209ea0c6818-road...
[2]: https://www.japantimes.co.jp/news/2019/10/04/national/kobe-e...