Abe was not supposed to be here, his speech was planned to happen in another city but instead it was decided to go to Nara the evening before. The killer couldn't have planned to kill Abe.
Why not? Wasn’t it public knowledge that Abe would be speaking there? At least with a few hours of notice?
In this case Shinzo Abe was doing some campaigning for his party, so obviously his destination must be communicated beforehand on the news so that people know to show up.