Even when his Uncle was in the mirror illusion and Cardan was mocking him, Aleister could have easily killed him but he could not bring himself to hate Cardan.
Not when he saw so much of himself in the man.
They were both similar in some way. Ruined and carved by the circumstances.
Growing up without a mother, and with a neglected father who banished him to the Enchantment Forest, it turned him into something he never was.
Alexander might have been his Uncle but the childhood he spent believing that his father despised him, and the circumstances he was cornered into carved him into a different person. He made decisions that never made him a good person.
Just like Cardan.
The sufferings of the people in the past, be it his mother, Cardan's mother, or their father... all of it was somehow molding their lives.