It's unfortunately auto-translated but it's the most in-depth article I've seen so far.

tl;dr
The assassin was angry at the Moon Sect/Unification Church for ruining his familty. His mother gave everything they had to the sect to the point where they didn't have enough for food.

The sect is notorious for preying on Japanese women to bleed them dry financially and marry them to Korean men.

He couldn't get close to officials of the sect so he went after Abe who had previously endorsed them.

It's unfortunately auto-translated but it's the most in-depth article I've seen so far.

tl;dr
The assassin was angry at the Moon Sect/Unification Church for ruining his familty. The sect is notorious for preying on Japanese women to bleed them dry financially and marry them to Korean men. He couldn't get close to officials of the sect so he went after Abe who had previously endorsed them.