I sure hope he doesn't let go. In most cases an edge-MC returns to his senses or has a change of heart, it's handled absolutely blah. Him being a brutal and inhuman arsehole is the central core of the plot. And if it is becoming dry, then it's because the scenarios have been too dull. I know it might be easier let him change the way he acts to give the story a "new" turn (not new in the genre at all), but afterwards it would be heading to generic town.
So, to let it stay more "unique" then other mangas with a revenge plot at the beginning, let him see this revenge through. (Yehh, I knopw it's not that unique, but the number of stories the "hero" becomes a regular hero is much higher, than those, in which she/he doesn't change.)