Slimmer, not impossible. Anyways, doesn't Japan have a history of hilariously brazen publoc assassinations? There was that one dude ran down with a sword on television...

Tetsuya's family got fucked over by Unification Church, and Abe's public support for them just rubbed that on his face and gave Tetsuya a physical target to release his frustrations upon.

It takes a lot of hatred to get around so many laws to fabricate an electrically triggered blackpowder firearm and fucking off a former Prime Minister in public.

More merciful now to sentence Tetsuya of a capital crime and execute him, as his life ended years ago, and its just been a husk of anger walking through the remains of that life.

This is as dry a case as you can get; the only variable is establishing if Tetsuya did everything alone, or if there was a support group or even a grooming agency that prodded him into this action.