What I’m about to say might be considered a major spoiler for you so WARNING!
From my understanding of the novel, Claude actually uses black magic to gradually erase Diana from his memories in the first timeline. That’s why, when he meets the 9 year old Athy, he’s already completely forgotten Diana by then, and at that point, he just sees her as a nuisance and a literal headache (because remember, Jeanette helps soothe the pain of his dark magic with her own while Athy just brings more trouble due to her being the key instrument to remembering (breaking the magic which is bound to bring horrific physical pain)).
As for the second timeline, due to Athy and Claude meeting much earlier than anticipated, Claude remembers Diana’s existence and the fact that Athanasia is their child much more quickly, and the dark magic is broken pretty easily (I’m guessing before it could take its full effect).
That being said, this is where the hate that you mentioned comes in — yes at first, he does blame her for Diana’s death and was actually planning on killing her REALLY early on, but in the end, he couldn’t bring himself to do it because even though he does hold some hate for her in his heart, he really does treasure her in the end. Please feel free to correct me about anything (づ ̄ ³ ̄)づ
I hate naivety... And Jeanette is toooooo naive... Some manhua or manhwa I hate it because of these naivety (╬ ̄皿 ̄) they are fool.. They didn't even think of what they do... It was like 'what I did is right' .. I don't mind evil cause at least they know what they did... But naive people is dumb and stupid (︶︿︶)