I fckn knew it, hes gone too soon.. he would become a great alliance for MC

Does that mean when he died only his body perished? coz how else was he able to help mc like that.If he's part of the heart demon then shouldn't he be fighting mc.

But he too strong to be side character so killing him was the only choice

No they explained it was the remnant soul of him left in mc sword not part of the heart demon so he has a will of his own still and used it to help him but most likely to see his daughter again