Politically: what motives would the shooter have? I don't follow Japanese politics closely.
Also, people are economically in a pretty bad place here, borders are still closed with no sign of opening. It might be a long shot but Abenomics was all about making Japan a "service economy", that relied heavily on tourism. No tourists, no money. Maybe a link there? Maybe Yakuza? Maybe the guy was just totally crazy and had trouble getting mental health assistance (which is also hard to get in Japan).
Japan is basically a one party state, it's advertised as a democracy but it's not really. I guess people feel desperate and there's no sign of any healthy change on the horizon. I guess that might be part of it too.
Shinzo Abe was a leader of the LDP which is the dominate party with no sign of that changing within the next few decades. It could feel oppressive for people.
Condolences to his family and friends.