Something seemed to have changed in Qie Ranzhe the moment he met the Taoist priest's gaze. It was like he was already on the fence right at the border between crazy and normal and all it took was this to tip him over to the dark side. That is to say, Qie Ranzhe blackened faster than how long it took Anakin Skywalker to blackened to Darth Vader.
The speed at which he awakened the beast within was as fast as making instant potatoes. Just mix that shit in water and boom you have got fluffy potatoes for Thanksgiving dinner in an instant.
He lost everything the day he lost his family but he worked through the grief and picked up the pieces. Two decades later and someone tries to pull the same stunt so how could he let them live?