You can see from the previous chapter's flashback that Claude was initially very happy when he heard the news that Diane was pregnant but when things went wrong Claude had to make the heart wrenching decision of either saving Diane by not letting Athy be born or let Athy be born and let Diane die. As you can see from the flashback Claude was craving for someone to love him, he suffered so much betrayal and hatred throughout his growing years and then he met Diane. Having to lose someone you love like that was clearly not easy for him and so he couldn't help but put the blame on the baby. Clearly Claude is wrong for treating his own child with ignorance but we all saw character development, people change and that's all that matters now. Sorry for my long rant
i kinda dont get diane? like if i found out the father of my daughter completely ignored her for the first few years of her life and then acted cold towards her, id probably kick him in the balls. idk why but i still hate claude (this is coming from someone who hated jeannette with a burning passion to liking her)