Responses
I think Dali thought he obligated to dote on Ul. Ul's actual mother who is human saved Dali twice. Dali suspected to be Ul's father but he isn't actually Ul's father because Ul's actual father is Angelico's biological father. Frieda wants Dali to adopt Ul and her last will is to Dali to love their sons. Raphael is Dali's biological son so he might thought he doesn't really need to prove his love.
I get what he does is super fucked up by biting sophie but this was all cause of the weird brainwashing/ need for affection from his dad. It was so sad that his last thought was literally his dad as well. Literally Ul and Raphael would've been alive if their dad wasn't so against mingling half-bloods and tried to force those beliefs onto his kids