I feel like Angelus represented the boyfriend who turned bad. A bit simplistic but ultimately Buffy slept with him and his behaviour changed. Afterwards he becomes the perfect stalker ex. It wasn't just those things that made me think he was the scariest but in general he hurt the Scoobies the most (physically and emotionally) and he had the background with them to make it worse. He had the knowledge and the inclination to destroy them psychologically. Even though I am male, Angelus' actions could have affected anybody. The best moment of Buffy's life resulted in her worst mistake.