At a few points during the season, The Walking Dead dropped subtle hints that Fat Joseph was responsible for keeping walker hordes away from the various communities with an explosion-focused strategy, and now that he’s dead, there might not be anyone to continue that plan, which won’t be good for anyone.
He’s ready to kill everybody, something he proved rather quickly once he escaped the Sanctuary’s walls.
Without much of a second thought, he channeled his inner Negan and completely bashed in the brains of the possibly non-evil Fat Joseph, who was pleading for his life at the time.