There were many factors Aries fell in love with Abel. He terrified her, yes, but aside from the fear of what he could do, everything still boiled down to what he had done for her. Abel might not be the kindest and could slay a man without a conscience, but what mattered to Aries was how he treated her.
One of the many things she liked about Abel was that, after coming clean about what he was, he hadn't kept Aries in the dark. He would openly say what was in his head if she asked him, even if he knew it might cause a stir in her heart.
And because of this, Aries came to know about the person Sunny brought into the empire.