Becoming a Death Eater wasn't her first choice. There was still enough of the idealistic witch inside her after she was removed from her broom cupboard that she couldn't imagine that she would ever fall completely in line with the violent beings she suddenly become surrounded by. When she was first inside of Antonin's house, his training was not to mold her into an effective, loyal Death Eater. That was the furthest idea from his mind and his master's mind. His orders were to break her down to be more malleable. If she remained as rigid in her ideals and convictions as she had been before Harry's defeat, she would've cracked and crumbled into a billion pieces. The new world wasn't built for someone that rigid.