
I'm not making excuses for Theo when I say this; Liliy has been through hell all her life and that's established but at some point she began lashing out on Theodore just because he's the closest person she could blame. In suffering it's hard to understand others and weight issues so I don't blame her too much.
He lost his entire family, his blood and flesh to her "family" so I understand distancing himself from her from the beginning. Mind you he's very young too