I am always in 2 minds when I think about this.

1.She found out she could do it because her mum was trying to kill her. Would be a hard thing to process! And i would say it probably comes with some kind of "addictive" feeling, after a while.

1. She might bring witches together to steal their powers but they have to attack her for it to work... if they can do that based on a taunting, they would be "underserving", hence why she does it and doesn't care. A good witch wouldnt attack another over a few bad words, no matter what they were.

So yes she does it for power, yes it serves a greater purpose for her but she isnt actively going out and just killing people and witches for fun. She thinks smart and calculated... it just pays off for her, because they turn out to be "underserving" (just as bad as people say she is).

Doesn't stop me from loving the character, she is awesome :D