Morally speaking, Lu Li could justify his actions. The four forces wouldn't make trouble openly as long as Lu Li didn't kill the five and stated his piece.
But no open trouble didn't mean no trouble at all. The four forces could take actions behind Lu Li's back. They could also delay their revenge.
Any one of the four forces could overwhelm the Northern Desert easily. Therefore, if Lu Li hoped for long term peace, he must first turn the whole thing into a bigger mess.
Instead of leaving right away, Lu Li had named his conditions, which was for the Grand Master of Demon-slaying Hall to take over the matter or he would kill Ji Mengtian, Yang Xuan and the rest. This was Lu Li's way of making the situation more serious.
He had forced himself into a tight corner. Either they would all perish together or he could gain long term peace. At the very least, he had bought himself a buffering time.
