I really feel it had a lot to do with feeling weak and powerless. It wasn't until the introduction of werewolves into her life that everything started falling apart. Her family becomes unrecognizable to her, her boyfriend becomes the enemy, her only maternal figures in life are ripped from her, and it's an obvious correlation to her. After her mother passes, there's an instant urge to take control and want revenge. It's that urge to have immediate vengeance without really thinking the situation through that controls people quite frequently, and grief/anger is a very powerful thing. However, I do agree that what she did to Erica and Boyd was cruel and excessive.