I think we're gonna find out at some point that he actually didn't.
He really did kill the MC's mother but doesn't remember because he went there and killed anyone with horns, he didn't even know how she looked like and only finds about it when Mc tells him.
I don't remember actually seeing that. I only remember him being there. I'll go back and check, but I don't think the artist ever explicitly showed or stated that he actually did any of that, it was just the speculation of MC and what other people said.
I know this from the novel, and sorry if I spoiled this for you I didn't mean to.
It’s like this bitch got amnesia and forgot that he killed his MOTHER??? And also butchered his brothers MOTHER????