
I think its because Claude believed that Athanasia was the one who killed Diana and Claude felt betrayed because Diana chose baby Athy instead of him. I dont know if you remember when Athy dreamed of his parents talking and claude said to Diana why she chose that(athy) being thats been eating her life away than him

Well, I don’t know the real specifics but I believe it was because he got mad at her for dieting? Yah, Ik, it sounds a bit weird, but he loved her dearly when she was alive, and she died when she gave birth to Anesthasia, which is why he did not like her in the beginning. I’m only assuming this.
After realizing that she had died to to giving birth he wanted to kill the child that was left behind in spite, and wanted to get rid of the feeling of abandonment from Diana, so he (tried) despising her instead for it, if that makes sense.
If Claude loved anesthesia mother so much why did he call her a "wench" or attempt to kill his own daughter that his beloved left behind?