Once Kretos's fame spread from killing one of the five demon lords, Demon Empress Nimune, many demons started challenging him. Since no one knows how strong Kretos truly was, gave the other demons confidence. Some of the demons felt like defeating the Demon Empress wasn't such a big deal since in one on one combat she was the weakest of the five. The only reason most of the demons were afraid of her was because of the succubus's charm skill and her large army.
The news that Kretos dealt with the Demon Empress Nimune's army hasn't spread yet, so currently, the demons of the south felt like they had a shot in becoming the new demon lord. Of course, this turn of events brought a happy smile on Kretos's face.