Deus might not be smart like those geniuses, but it was by no means that he was dumb and ignorant. If he was, he would have already been dead because of being assassinated by the young master that he had offended.
He was smart enough to at least prevent himself from being killed by that young master, even if he was forced to lower his head. But now that he had power, he decided he would take revenge against that young master. Not only for himself, but also for his former girlfriend.
If he wasn't powerless, unlike before, he would have given his girlfriend a better life and that young master wouldn't have dared to offend him and his girlfriend. But since he now has power, he would take the matter into his own hands, and sooner or later, he would enact his revenge plan.
The reason that Deus couldn't do it before was that he lacked power and background, but now that he was a warrior with a high ranking status in the Warrior's Association, Deus was fearless, unlike before.
