Hate Her

I have never disliked someone as much.
There's a girl who I have only heard bad things about. She keeps telling one girl that no one wants her in their homecoming group and that she's being uninvited. The same girl she harrassed has a sunburn on her left cheek. Today, after a band show, she walks up to the girl and says, "Hey _____, you've got something on your face." BAM. She smacks her across the burned side of her face. HARD. And laughs.
The girl she smacked grabbed her cheek and gasped. Her eyes were tearing up.
Not done yet, the evil girl turns to a boy sitting nearby and says, "Hey, hit her left cheek."
At which point, I turned towards her and said, "Hey, don't be a-" BITCH. "Never mind."
I could tell she could hear it coming. But I stopped myself. Because I love and respect her parents and her sisters. And I looked at her and she looks like her sister, who I love.
What I should have said:
"Have some human decency. I have heard nothing good about you this year and you're living up to that image. And it's not flattering. You need to make a good impression when you're new in this school. I'm not impressed with how you act and you should really try to be decent. That's really all I ask."
But I didn't. I turned on my heel and left. I shouldn't have left.