I would place her Alignment as Chaotic Neutral. She is completely and blindly self-centered, to the extent that she interpreted her Husband's actions as truly 'stifling.'

She chose poison, not because she is 'evil' - her actions were really not intended to cause harm for the sake of causing harm, or because she held real malice for her husband, but rather she is so selfish that she simply chose the most expedious option to get what she wants - in this case it would be her husband's death, and when that failed, chose the next most expedious option, to run away.