I might be delusional but yk how yuder can never properly recall the memory of that night ? I don't think he killed past kishiar. Kishiar's awakened power is literally to push and pull objects and energies. It is very much possible that yuder wasnt able to carry out the assassination order and so kishiar killed himself via Yuder's hand.
Yeah kishiar did it himself but Yuder remembers his hands holding the weapons. And as of this moment in the manhwa, he didn't know Kishiar's power yet.
The reason why his memory is imperfect is because of their bond. When Kishiar severed their bond before his death, it messed up Yuder's memory. (Usually bonded partner will die after one of them passed away)
I cried so hard lol when Yuder thought why Nathan didn't avenge his master death. I'm so weak for first tl Kishiar. He must have ordered Nathan to not do anything to Yuder and to leave him alone on that night Yuder came to execute the order to kill Kishiar.