This change had occurred too quickly.
Even the three venerables could not take action in time.
Their original plan was to wait for Spectral Soul Demon Venerable to go down with the sun, until they were both greatly weakened. Because Spectral Soul Demon Venerable was following his instincts now, he would naturally also want to survive. Thus, if he got badly injured, he would escape.
At this time, the three venerables would target the greatly weakened Spectral Soul Demon Venerable.
In the contest of three venerables, Spectral Soul Demon Venerable was a huge uncertainty. There was still a chance of him regaining his sanity, thus, the three venerables were in agreement that this uncertainty should be killed first.
Like during the battle in Crazed Demon Cave, Star Constellation Immortal Venerable and Giant Sun Immortal Venerable decided to cooperate to kill all the pseudo venerables first.