I'm only at E10 but it's probably the case that ML's father or the accomplice killed her. 

At E11 now, the accomplice (the real Baek Hee Sung) killed her on the orders of ML's father.

 martar:

Suddenly, after a few months, I realized I don't know what happend to ML's mother. I watched many episodes in korean, so maybe I didn't understand something. Can anyone tell me if she was murdered? Or is it mistery? 

Edit: Sorry for possible spelling mistakes, I'm not a native speaker

The father said he truly loved his wife, but his wife loved only an illusion, and that made him dissapointed and it triggered his murderous acts, killing his wife first thing, so basically the father became a serial killer after then and found a young boy (Baek Heesung) to accompany him since his real son wasn't capable of hurting anything!