I Feel Safe
That Minuha needed to be complete for him to truly kill her was something Seth had known right since when he interacted with her at the dungeon.
This was the reason why Seth hadn't even entertained the thought of killing her when she was incomplete and had the strength of a level 9 mortal.
After comprehending the basics of darkness, Seth had come to understand that Minuha was a completely different being, one far removed from humans and other common species.
Though he had comprehended darkness, Seth had no idea how to replicate Minuha's abilities with her shadows.
He could understand the basis on which her shadow ability worked, how it used darkness as a fuel, but he could never understand the essence on which she tuned to and from a shadow.
Taking Minuha as a great mystery, Seth decided to push aside her uniqueness and go about killing the woman, confident in being able to end her if he cut off the fuel for her unique movements.