Being playful was Nikolas' nature, despite his age and the experiences he had before his death. What he said about cultivation was just a joke, not his true thoughts about the cultivation.
As for why he killed the God Realm in one shot? That was never his intention. He didn't know how strong the God Realm was, and he could not possibly guess how much power he needed to adjust not to kill that person. Since he didn't know how strong the God Realm cultivator was, he just shot one of his powerful skills.
That killed the person in one shot, killing the people who foolishly stood behind him. That one strike killed more than fifty people.
Nikolas then turned back to the enemy after that shout, scanning them with a frown. He didn't have Detection skill, so he must use his instinct to pick up his next opponent.
