Anyone who knows even a little history will realize that the aftermath of an assassination of a public figure results in an immediate increase in the popularity

Anyone who knows even a little history will realize that the aftermath of an assassination of a public figure results in an immediate increase in the popularity of the affiliated political party (the 'martyr effect'). The claims that the assassination of JFK in the United States in 1963 changed official US government policy are nonsense, incidentally - JFK was an ardent Cold Warrior who would have followed the same policy Johnson did, if faced with the imminent collapse of the South Vietnamese government. If anything the assassination strengthened the Democratic Party's political position at the time, and the same kind of effect seems to be taking place in Japan, with Abe's party expected to win a major victory. The story of an individual with personal grievances (possibly acting alone, possibly not) related to a parent who went bankrupt by donating all their money to a religious organization affilated with the victim makes much more sense in terms of a plausible motivation. The other part of the puzzle is why security for such a well-known public figure was so poor, but Japan's history of low gun violence might explain that.