Maybe because he was way too op?
But how the hell kisiar died!?
From my understanding his mana was destroyed, taken away or something like that. Mana is like energy so they can use their power, so if a person is out of mana they can't use their power. That's why he can't use his power and cannot fight back. Also remember when he saw that blue haired guy that's in charge when his mana is taken away or destroyed, someone like that, his expression become dark? It's because of that.
Anyway, this explanation maybe a bit confusing but I hope you get my point or you can just reread the story
Ah....not really...ML's powers become unbalanced. As i said he is too op that's why his powers needs to have constant balance so he won't die. In his last life something happens (which is spoiler so I won't mention) and because of that incident it causes imbalance in his body. I won't say how many or how much he is powerful since it will be major spoiler but it has nothing to do with mana. And it has been stated that Yuder killed ML in his last life (even though he was in verge of dying anyway)
it was a rhetorical question but thanks babez
the hell ml is op asf how the hell did he die