Firstly, I'm a bit doubtful that the Order would give such a young person such a brutal task. If they must kill someone, then they would leave it to the adults, not the young people.
If you insist, then Ginny would say no. She's not a killer, no matter how horrible the person is. Ginny wouldn't be able to kill a person and she knows that. She would be shocked and surprised at the assignment, and would definately not accept it.
This is the most likely scenario of what would happen, I'm sorry that it goes against your wishes.