Because he is an incredibly seductive and desirable yet quit a flawed male lead character.
Not only is educated, attractive, socially and sexually dominant: he has lived for more than a century and inherited his family estate, he had time to gathered money. Above all those things he sees himself as a monster, his looks makes him irrestible to humans being wich is a good way to lure his victims and because of this he views himself as a soulless monster wich is why he behaves so unfriendly towards Bella who loves him but can only be with him by becoming like him, that's why he hates himself. The woman he loves has to become a monster just to be with him. This can explain his bipolar behavior towards Bella. When she becomes pregnant to his child, she has to endure unbelievabel pain and death.