Although Vaan felt confident the elusive demon-worshipping group had Hecate's blessings and knowledge, he couldn't be completely certain. After all, far too little was known about the Seven Great Devils.
Thus, how could he easily guess the thoughts and choices of a Great Devil with his limited understanding?
Moreover, according to some historical books, Hecate the Mystical was also known to be the most enigmatic and eccentric among the Seven Great Devils.
It was said that in Gehenna's long years of existence, Hecate the Mystical had shown three faces. Sometimes, she was evil. Sometimes, she was good. And sometimes, she was just neutral in her judgment.
Her change of nature had no rhythm; she was simply unpredictable.
That said, Hecate had shown little-to-no malice towards humanity in the past three hundred years.