The ML could've been cursed by or for the people he killed after the MC's OG death??
Or he deliberately killed so many of his people, because he either was told or deluded himself into believing it would bring the MC back to life?
Very excited(?) Feels wrong to use that word, but ... to see how or why the OG MC became 'monstrous' or 'evil' that the ML felt they had no choice but to kill him ლ(´ڡ`ლ)
Definitely not a Shonen. A cute Shoujo, with not reincarnation.