Assuming Hee Sung is really the accomplice and Mum found out that night, it is understandable why she might have gone ballistic and "hurt" him. She gave birth to him, raise and nurture him — gave him the best (and don't forget from the argument it seems like the husband was not around most of the time) only to find out he is a serial killer. How heartbroken it must have been. Her world must have crumbled at that very moment. All that love she put in and she's told she has raised a Devil...? This is from a mother's perspective.
From Hee Sung's perspective perhaps the love and expectation (being a Maths genius) was the reason Hee Sung went haywire. That's why Mum went berserk on Eun Ha. She didn't want her to become broken from overly good at something. She realises the "mistake".