For example, you are at a job you hate every day and your boss is a moron who likes to be a jerk. You put up with this. Some people don't. They react from stress in the worst way and kill the people that cause their stress, or just kill bystanders. Maybe they are still children.
People are rude.
Most people don't know the limits, they don't realize they are intruding on someone's territory. There are so many people that most people don't even have personal space. People need their space. I know someone out there in the audience will raise their hand and say that it is crowded in Japan and people put pressure on their kids in Japan and they aren't shooting each other all the time like they do here. I will say, I don't care, I have never been to Japan, and I don't live in Japan.