As much sympathy as I have for Regina, it does not lesson the fact that numerous people have gotten hurt. True, her activating the curse was due to manipulation, but she also had a choice. Her father said she should forget about Snow White, about power, about her revenge, and come with him to have happiness. She might have been happy like that–she will not never know for certain. 🙁
Maleficent also warned Regina of the consequences–Regina still pursued her goal, despite the warnings.
In many ways, Regina acts like a child in need of love and in need of fairness (in a childish way). The way she talked to her father before she killed him showed that, as well as her confrontation with Emma in the closet. Her friendship with Kathryn (before the latter yelled at her) also shows that. But with people like Kathryn and her father, once they betray or reject her in anyway that she percieves to be unacceptable, then they are not safe.
With Henry, it can be argued that there is an exception, but it might also be that Henry (in her eyes) goes too far.