Villain Motivations

I think that she sounds more like an anti-hero than a villian. I think it would be that she will do whatever she can to get back at Dr. Eggman, and doesn't really care about who she has to destroy to get back at him. And eventually she would get back at other people doing experiements on other people/animals, still not caring who she has to kill to take them down.

Main motivation would probably be revenge.

 

So she goes out and screws with other people in a generally violent way? Correct?

 

If she has no care for the lives of others, and simply enjoys ruining other people's days, you could take that to an interesting psychological aspect. As a test subject, she probably experienced pretty constant and pretty high levels of pain. Because of this, she could be inflicting pain upon others as a way to rationalize her own pain. Hurting others makes her feel better about her own time as a test subject. Or, as a mildly darker aspect, she could be hurting others to connect emotionally with them. She has experienced unimaginable pain, so inflicting pain on others makes them seem more like her and more understandable in her own mind. That second option is where you get very dark characters, ones that smile with true joy all the while they inflict pain, because they believe that what they are doing is right and helpful.

It also could be partial regression due to emotional trauma and repressed memories from her time as a test subject, combined with her shutting down her feeling of empathy to save herself from her testing. That would cause her to be much more quick to swing between emotions, and make her as cold as ice.

Just some suggestions.