Inside Atermiss house, both Artemis and Mark had a serious conversation. The conversation was related to the past of Mark, which Mark wasn too open about, but still open enough to tell Artemis the details that she needed to know.
What Mark told her, was the basic information such as the murderer of his mother. As for the reason why, and how exactly she was murdered or treated, that he did not explain. There was no need for him to explain that too, since it is a sensitive and private subject, and because it is quite unnecessary for Artemis to know about all that.