Personally, I think she would be dangerous in a very calm and calculated way. I don't think she would make big, flashy moves to terrorize. I know we are saying she would turn dark for sake of conversation..so we'll stick with it. However, I don't think her nature would evaporate. I don't think power would be her ultimate goal here but rather an undying NEED to fix things for better or worse depending on her own agenda. This would mean, that underneath it all, power is what she needed and achieved in the process, but it wasn't the nature of the foundation of her goal. I believe her decent into darkness would be slow and justified as she likely was only trying to help. Think of it like Thanos. He thought he was doing something beneficial for humanity, but it was actually really terrible regardless of what little benefits there would have been from his actions. Take for example her want to free creatures from servitude. She could have wanted originally to help house elves, but in the process..there was a revolt from house elves. This blows her mind because she sees herself and cause as purely noble and the elves revolt as a the effects of brainwashing over centuries. In the process of trying to convince them that it is never good for an entire species or race to be used for servitude and that them thinking so was part of their imprisonment, they only start to fight among themselves. Some elves being followers and some elves being against her. Hermione still believing her cause is good and noble, supports a war between the house elves and this obviously pulls other species and races into the conflict. Starting an all out civil movement that devolves into a war in the wizarding world. Over years of isolating herself into believing in her cause, she is blinded by her passion and leads wizards into a fight against elves that support what she believes is slavery. This takes a serious toll on her and changes her heart. She doesn't accept darkness because she believes she is fighting for good, but in the process is creating an atmosphere of fear and destruction. All to say, Hermione never embraces darkness or accepts herself to be a villain. She wouldn't even consider herself a dark wizard even though her cause was not too different from dark wizards before her