Compared to Qin Jue and Killing Dao, Shi Tian was really too weak. He was only at the late-phase of the Supreme Stage. In the Southern Land of the Spirit Central World, he might be considered an expert, but in the God Realm, he couldn't even be considered an "ant". Any itinerant cultivator could easily crush him, so it wasn't bad for him to stay in a small world to cultivate.
In addition, to be safe, Qin Jue deliberately left a mark on Shi Tian's body. This mark could be sensed by living beings in small worlds to prevent Shi Tian from being attacked.
After all, many fiend beasts in small worlds were Great Sage Stage and Great Void Stage experts. They were not existences that Shi Tian could contend with.
Putting away the small world, Qin Jue didn't say anything else. He turned around and flew out of the mysterious space with Killing Dao, quickly disappearing into the horizon.