Chi Xue and Po Tian would remain a threat to Ling Tian, Hua Min'er, and the others as long as they were alive. This was especially true if, in the future, they managed to find Primordial Chaos Qi ahead of time and help the Lord of the Universe complete his mission. After all, if that happened, the Lord of the Universe would no longer need to make a "deal" with Ling Tian. He might not go so far as to kill Hua Min'er and the others outright, but he wouldn't be "kind" enough to release them from the universe, either.
Unfortunately, Ling Tian knew that he had almost no chance of killing Chi Xue and Po Tian in the God Realm, because the Lord of the Universe would prevent it. After all, Chi Xue and Po Tian were also seeds chosen by the Lord of the Universe. Even if they were only backups, they would still have a chance to help him find Primordial Chaos Qi and complete his mission after breaking free of his control. Under the circumstances, he wouldn't allow Ling Tian to kill them.
The Lord of the Universe could influence what happened in the God Realm, but he couldn't control what happened beyond it. If Ling Tian encountered Chi Xue, Po Tian, or the others beyond the universe, he wouldn't have so many qualms and could kill them outright. Most importantly, both Ling Tian and Little Devourer had the strength to kill Chi Xue and the others—and to do it with ease.
"Hopefully, once you break free of the Lord of the Universe's control, you and Little Devourer will run into Chi Xue and the others," Po Qiong muttered to himself. "Heh, with your strength and Little Devourer's, if you get the chance to meet them