Chen Guang, having coincidentally acquired the Heaven Demon Order from the Prison Subduing Demon Sect, had since kept it a secret.
Who could resist the temptation of the inheritance of a Sect-Defending Technique from the Demonic Sect, one of the three paths of Jianghu, which had swept across the world in its heyday?
With his perspective, he wouldn't be limited by the conventional distinction between righteous and evil.
He understood better than some lowly folks in Jianghu that Cultivation Techniques simply needed to point directly to the origin and allow transcendence.
As for which path to take, that was of secondary importance.
The saying goes, "The victors are kings, and the losers bandits."
This is true for mundane power struggles as for the contention within Immortal Sects.
Those who lose are naturally deemed demons, and those who win represent the righteous path of the world; they are immortals, they are Buddhas.