Spoilers
Yuder killed Kishiar in the last because Katchian (the new emperor at the time) felt like Kishiar was going to try to overthrow him. Therefore, the emperor Katchian decided to dispatch the current commander, Yuder, to take out this potential threat. Basically Yuder was following orders.
Important!!: Please keep in mind that Yuder's past life memory is fragmented and more information will be available as he remembers more.
This is as far as I know from the free novel chapters available. I believe in the paid chapters (which are ahead by a few hundred) it is revealed that Yuder couldn't actually bring himself to deliver the fatal blow and Kishiar, using his awakened ability, pulls Yuder's sword into his chest, ultimately killing himself.
Happy reading and have a wonderful rest of your day!ヾ(☆▽☆)
thank you so much for answering my question!!! (*☻-☻*) you have a great day ahead too!
Could someone spoil me as to why yuder/yuther killed the commander in the past?