I don't think he'll purposely kill her. I think that was the look of him losing all trust, but i don't think he'll kill her on purpose. He's so devastated BECAUSE she means so much to him. He's feeling betrayal. He was trying to deny it even to himself that she was poisoning him. I think she'll go crazy and attack him when he calls her out and she'll accidentally die. That or she'll just leave.
I think the mother will soon be killed by Sangwoo. He had the same look as her, he became totally crazy it's obvious! (⊙…⊙ )
I don't think he'll purposely kill her. I think that was the look of him losing all trust, but i don't think he'll kill her on purpose. He's so devastated BECAUSE she means so much to him. He's feeling betrayal. He was trying to deny it even to himself that she was poisoning him. I think she'll go crazy and attack him when he calls her out and she'll accidentally die. That or she'll just leave.