Gurl.... Wrong theory..... Kishiar was never evil and no the black mark doesn't make you turn evil. Yes, Yuder had to kill Kishiar in their past life but that's for a whole different reason.
The reason how Yuder was investigating the end of the world is because of the monsters appearing and the tear in the space. Well good try tho but I just feel like I need to correct your guess sorry...
Its okay... that is why I said it was a theory. It didn't need to be perfect and I didn't intend for it to be correct. I haven't read the novel so of course there may be people who know the real story. Thanks tho.
Ur welcome... I really recommend you to read the novel tho it's really good. Istg you won't regret it
Spoilers (my theory so far)
So I am guessing the black mark that he got in this life was inflicted to the commander in the past life. He was turning evil or something that is why he said he regreted understimating getting hurt by the red stone. At the end the black haired guy had to kill him or something nd that is how he knew that the world was about to get destroyed in the past life?