Of the three killing spells, the first two, Finger of Death and Demonic Finger, were often used by Wang Lin. Their power wasn’t enough to threaten Ascendant cultivators and could only be used to suppress Soul Transformation cultivators.
But when Situ Nan used the Finger of Death and Demonic Finger, their power was shocking; even Ascendant cultivators would be killed by them. Aside from the difference in cultivation levels between Wang Lin and Situ Nan, there were also the essences of the Finger of Death and Demonic Finger.
These three killing spells were created by Situ Nan, so of course they were of demonic nature. Unless one had a demonic heart, it would be impossible to learn the essences.
And Wang Lin’s dao was very different from Situ Nan’s dao. Thus, when Wang Lin used these two spells, they lacked change and flexibility.
In truth, there were follow up spells to these two spells, but because Situ Na noticed their differences, he didn’t teach them to Wang Lin.