The thing is, it wasn't even his actual mom. He was adopted, abused, molested and tortured by his mother, if not her abusive boyfriend. He had read everything there was to know, whether it was lying about him being her real son. His mother was manipulative and terrible to him in life, but he's had what I'd say is Stockholm syndrome and is used to obeying his mom all the time, despite the terrible pains he's brought into his life, and for him to snap after finding out the truth makes it that much more understandable and sympathetic to an extent. He's devoted almost his entire life living and supporting his mother despite her manipulation and infliction of pain and misery in Arthur's life, because that's what sons do, but what he found out was that he was born, adopted and since been abused to the point of a child service record. To be born a bastard, given away to a terrible family, who you then take care of under the assumption they're your family and you love them just for that, and then center your life around their care, only to find out that the whole time, they're strangers who took on a child and abused and tortured them. That's a sick joke.