TL;DR

Shinzo Abe was associated with a religious group 'Church of Unification'... that motivated Tetsuya Yamagami to target and kill Shinzo Abe.

Yamagami told investigators that he blamed a “religious group” (i.e. the Unification Church) for inducing his mother to donate so much money that it bankrupted the family. He initially prepared to kill a church leader, but gave up after deciding it was too difficult. Yamagami then turned his focus on Abe, whom he was aware promoted the church at some level.