ヾ(☆▽☆) but the question is we saw his parents.. And i don't remember that the auther mentioned if he was adopted.. So maybe he sneaked and saw our baby dying in forest and as we know that the demon king is alittle unpredictable implied his life force.. And that would be more logically and non repeatable (づ ̄ ³ ̄)づ
Memories altercation or just like in the manhwa i read rn, he got reincarnated in a game where he had parents and all, after he entered the academy and raised the number of his connection with the skill he had, he lost EVERYTHING he learned about the body he now owned. Akd realized that, maybe they were just used so that he could adapt to the life he now had.
Maybe it's the same with iruma. He had parents in memories but those arent his parents, but rather people that were chosen to keep iruma safe but decided to ruin it.

May contain spoiler
Sooo.. I feel like the demon king sneaked had a baby and thought that 'that' would happen to the baby and tge mother.. And the baby was dying after his mother so he implied his life force into him an tadaaaa ヾ(☆▽☆)