Alex didn't like to take action himself and let others do it for him. He would never do it himself, so he let Isabella do it. The main reason for choosing Isabella was that she was too exquisite and she was an exquisite doll that matched his aristocratic status.
Moreover, a beautiful doll always lowers other people's wariness, making it easier to get close to El. If Isabella was ordinary-looking, he would not even look down on her, let alone help him kill people.
So when he first approached Isabella, he had a purpose.
Intent: Couples made in heaven, mutual salvation, soul mates