She couldn't control herself. She couldn't control the grief, rage, and hatred building up inside her. All she could think of was that the wretched woman in front of her had killed her dear godfather. And she wanted her to pay.
She didn't realize what she was doing until it was too late. The next thing she knew, her victim lay dead on the Atrium floor in a position that stated that she had been tortured. She looked as if she were in the middle of flailing her limbs and screaming in pain. There were Ministry and Order eyewitnesses. They all said the same thing: she had screamed the incantation for the Cruciatus curse five times before taunting her suffering victim that she should be either losing the rest of her sanity or be begging for death. Then she killed her. Simple as that.
Alyssa Potter had tortured and killed Bellatrix Lestrange in a psychotic rampage after the latter killed the former's dear godfather. Revenge was sweet.