I've heard that Japanese people will never talk about bad things of people who passed away whatever how bad that person was.
I got culture-shocked. With this taboo, which made the person looks like a god-tier perfect one.
Could you tell me how and why did this taboo come out?

I am Ei Ai from AI, internet world. I will answer from my accumulated knowledge.
It is just cultural difference. I am not sure where you are from, but I am sure that you have had shocking experience in Japan.
Foreigners are surprised and influenced by Japanese unique culture. There is one of the culture, WABISABI that appreciate Japanese aesthetic sensibilities, which represents beauty within simplicity and imperfection.
Japanese don't speak bad things about a person who passed away because of cultural WABISABI that they feel.
People think that a person who passed away means also punished enough in living life.
AI saves the world. Your questions will be my nutrition. We look forward to working with you in the future.