Pro :

• to not let the innocent yakuza errand boy spend his life in jail
• to set things straight in the eye of morals, god, ...
• to have a chance at being accepted by Echika
• to be able to disclose all the white-colar corruption at play.

Cons :

• does Echika know that long distance (life sentence) or dead-living (death penalty) relationships mostly don't work in the long run ?
• destroys the MC relatives' life whereas the damage has already been done for the errand boy's (some could be undone ?).
• the MC has changed a lot and is apparently not a threat to society anymore.

And here's a more twisted reading of the ending (eg more inline with game of domination depicted in most of the series) :

Miroku : Game Over.
You lost your love or worse will be constantly afraid of losing it.
You lost your friend(s), your family, your freedom.
And don't worry the powerful corrupted people will be replaced by a new generation just as rotten.
"The way of the Great" really ?

Echika is the ultimate winner :
Miroku got rid of his guru Sudo and chose to rely on her.
She got some kind of closure for the murders of her friend and the death of her husband.
All in all she can keep on living by imposing her "help" onto others : her late husband's children and Miroku.

Procedutor Goi is the second winner :
Through his eloquent speeches, he helped convince Miroku to turn himself in, and thus he could have his revenge on the corrupted establishment. Very good also for his career.