The mom is more mentally unstable then we realise,
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some raws I came across before show a flashback of when Kyunghee was a child. His mother is younger, distraught in their kitchen and Kyunghee who's a little boy at this point tries to cheers her up by washing dishes. The sad thing is he has what looks a bruise on his face, possibily from her hitting him.
I'm hopping this situation doesn't end in a murder suicide. I don't thing she could survive killing her son in a fit of passion.
Mentioned
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http://m.pc18.com/info/31888.html
The mom.... THE MOM.